Wondering what you guys are using for holding boards while soldering?

I have been using a Harbor Freight panavise knockoff for years
https://www.harborfreight.com/2-3-4-quarter-inch-articulated-vacuum-vise-3311.html

The vacuum feature never really worked, but while it gets the job done, I'm wondering what else is out there?

Looked at Panavise options. They look nice, but get a little pricey.
Then there are the helping hands things with alligator clips on flexible arms.
And these little clamp things

So any advice or suggestions appreciated.

Randy

Dear Community,



I need some help and your advice will be very appreciated.

At the moment, I am using the RPI 4 as end point. The Khadas tone board is connect with the RPI 4 by USB Khadas cable found in the box. In order to improve the sound (actually I am happy with the RPI + DAC but I would like to make some experiment), I want to power the KTB using the GPIO connection. My question are:

1) If I use the GPIO to power the KTB, can I sill use the USB-C just (link to RPI) to input the signal?

2) If I use the GPIO to power the KTB and the USB-C (link to RPI) just to input the signal, should the USB cable be without +5V connection (red wire cut) in order to avoid a double power supply or useless connection (which can introduce noise)?

3) Could you suggest a power supply you know is good in your experience, without sell the house? Is the IFI Ipower 5V a good solution? Do you have a better one?

4) Please could you suggest a good USB cable in your experience? without sell the car..
5) Do you know if USB cable has to have demanding 90ohm +-15%? Or less is better?

Thank you so much in advance. Thank you for the attention.



Kind Regards,

Francesco

I m making a 2 channel amplifier that has 2 wires for each channel for the input that is used for RCA. Now I also want to use XLR along with RCA inputs but I’m not sure on the XLR as it has 3 wires Hot, Cold and Ground, how would the wire to a amplifiers board that has only 2 inputs for 2 wires not 3?

For Sale: HP 3580A Spectrum Analyzer

Good condition, no broken knobs, all functions work fine. Chip in upper left corner of reticule glass, but not in usable area and doesn't effect functionality. User manual on CD if you would like, but its available on the web for free.

I've been using it regularly for about five years with no complaints, but I got a good deal on an HP3589A.

$300 USD + shipping (UPS, ~35 lbs). Paypal or cashiers check only. PM if interested.
Bill

I have a pair of Bextrene cone laminated 8 inch bass driver taken from a pair of B&W DM2.
This is a replacement driver stamped May 1993 and not the original DW200/2 bass driver designed by B&W the 70's. Limited play in the cone can be observed as the coil tube hits the back plate. On my Sansui 881 it hits the back magnet at around half the volume making horrible noises.
I would be interested if it would be worth the effort to refurbish these by doing the following upgrades:
1. Replace the rubber surround with a rubber surround that has a wider bubble / bulge so that it allows for more flexibility and potentially bass. Original driver DW200/4 has wider bubble / bulge by 5mm as a consequence it does deliver more bass than this one.
2. Remove the back plate and add a metal ring between the magnet and the back plate that would leave more space for the core tube to move at the back. I'm only thinking of refurbishing these as I want to get more power out of the DM2's and these have a 700g heavier magnet than the DW200/2 original driver made by B&W in the 70's.
Has anyone attempted to do such extensive refurb. Would I risk damaging the magnet by trying to remove the back plate?

https://youtu.be/MIXDwRNceNo

Hi everyone!
I'm using Li-Ion battery supply for a headphone amplifier. I would like to replace the batteries with a very good linear regulator with minimal noise. I need max. 0.7A output. I have a 10VA toroidal transformer with a 9VAC output and was planning to use that if possible. But I have no clue as how to find a very good regulator suitable for this. Ideally something I could buy already assembled or just a circuit diagram so I can assemble one (I'm not an electronics designer but I can handle a solder iron) 🙂

Hi All,
I need a fast & big (15-18" driver) sub for my home theater setup only for LFE use with movies to match my Pi Theater 4 LRs. And, yes, as cheap as possible since i will not use it for music bit just for cine-fun 😀
Suggestions?
Thanks

I have an older Infinity Kappa setup in my car, the front has 60.11CS 2way component set

I ordered a set of 20mx with their included bandpass crossovers thinking I could add them on, but upon further research, it turns out they are designed to plug into the newer 60csx crossovers which have some kind of switch to tell them the mid system is attached, but I can't find any technical info on it.

Should I just run the mids on their own channel off the head unit? If I were to run them on the same amplified channel as the components, could I parallel it to the woofers leads (which already passed through their crossovers) or I have to parallel them before their crossovers?

I'll post some pics so others can see this 20mx crossover

It seems they are obsolete everywhere I look. Are there any substitutes?
Thanks!

Hello all, I’m really needing assistance in finding replacements (equal to or superior) for MTP8N10 and MTP8P10 n and p channel mosfets.

I picked up a fairly rare amplifier, a Superphon Maxx 806 which was designed by Stan Warren of PS audio. Unfortunately it’s so rare that I can’t find any service or schematic info on it anywhere. It uses two of each in the output of each channel, so the amp has four of the N channel and four of the P. One channel has had the outputs destroyed.

Before I opened the amp I saw that they were TO-220 device and was praying for BJTs, but no luck. I know that matching for fets can be trickier and I’m having no luck finding cross reference online. I was hoping for maybe RFP8N10, but no.

Can anyone point out available devices that should work as drop in replacements for these two devices? Hopefully someone is familiar with them as I can’t find much info on them at all.


I really appreciate any help.

Dan

After unintentionally hijacking the Sony VFET Part 2 thread asking for opinions
on speakers that excel with Papa's amps, I've decided to ask my questions in a new thread.

What speaker design do you find most satisfying and/or revealing for our beloved 25 watt Pass amps?

Is your amp SE or PP or do you have both?

Do you prefer a 3 way, 2 way, passive, biamped, or full range?

OB or box?

Ancient technology or new high tech drivers?

I'm especially interested in hearing impressions from those who have experienced
different combinations of the above to see if there are common threads that arise.

The following and posts 2-18 were kindly moved by the moderator from the VFET thread to this new thread:

wdecho said:

Your english is very good being it is your second language.

Click to expand...

+1

Proton76, your english is much better than my ________. (Fill in the blank) 😀

Where do you cross over your drivers?

And yes, thanks for sharing info on your speaks. My next challenge is to decide on a set of speakers that are efficient, and worthy of my Pass amps!

Hello,
I'm thinking about making a stereo tube amplifier. I found Dynaco projects; mark 3 (60W) and stereo 70 mk2 (by Daniel Steele).
I would like to have about 50W output. MK3 is higher, but more expensive. And ST70 is only 35W, but cheaper. Maybe there is an option to increase ST70 power by eg. 10W? https://www.bonavolta.ch/hobby/en/audio/steele70.htm

And the second thing: the output transformer. I have a possibility to buy A470 transformer replicas (for ST70) or Hammond 1650NA (will be good for MK3?).
Btw, As you might see, I'm new in tubes. And sorry for my english 😀

In my nick of the woods there's a lot of speculation on a Chinese move on Taiwan.
I don't want to discuss that as it's against the rules.
But if there is a real chance, should we stock up on chips?
If you had a war chest of $ 500, which chips would you stock up on?

Jan

I have a like new hard tool case I am no longer using. Dimensions are 19"x9"x14". Have original key and combination instructions.

Asking $50 and can ship if you want to pay for shipping. It's not heavy. I do have a box ready too.

PM if interested.

I have a Tektronix 2710 Spectrum Analyzer that blows the fuse upon powering up. I did use this unit in the past, but no longer doing RF. Anyone interested in a parts unit or trying to repair?
PLEASE PM ME IF INTERESTED. I dont check the forum very often due to life obligations. If you PM, I can answer quickly.
Shipping from PA. I have shipped scopes and other test equipment to members here. It will be packaged well.

Thanks

Time to sell my beloved, excellent condition HP 3580A spectrum analyzer. This is a perfect condition unit with a later build date (1980 capacitor date codes). This unit also has option 002 for balanced input.

I picked it up locally a few years ago and used it to help learn about spectrum analyzing and learning to use FFT software. I now use FFT software exclusively and it is time for someone else to use this instrument.

The CRT is sharp, clear and bright. No burn and the store function works properly.

Comes with original manual.

Pictures show the front of the analyzer and also show the unit measuring a Stereo 70 with distortion.

Unit is available locally in South Central Pennsylvania. I can ship and I package very well.

I don't want to eBay the instrument, but I would also like to fund other projects.

Hit me up with offers. You all know what it is worth and I'm willing to work with you.

I'm building a KT88 PP amplifier from a PCB which has the valve bases soldered in. The chassis I'm using already has the correct number of valve base cutouts but they dont align with the PCB. I dnt want to mess with the chassis so my question is would it be okay to use the existing chassis cutouts for the valves and link the chassis valve holders to the PCB valve holders with short links of wires. The longest link would only be about 5cm. I'm no physicist so I dont know whether the signal paths, voltages or the valve performances would be be affected. The links are so short I can't imagine they would be but I bow to the superior wisdom of the much more knowledgeable members of this forum. The question may be dumb but any advice would be appreciated.🙂

hi I have a pair of above speakers I'd like to make new floorstanding cabinets for . if I increase the size of the box and keep the same size port how would this affect the tuning? would this just give me a lower f3 or would I be better keeping the internal volume the same and filling the bottom part of the box woth sand or something like that . reasons for building a floorsatng box is to make the appearance nicer for our new living room . the box would be better constructed as well with some nice veneer and some cement boards inside to hopefully get rid of any panel resonances
thansk in advance

Hi guys,

I'm looking for a schematic for the Music Reference RM-5 preamp.

If you have it could you PM me or post it here?

Thanks!

I have a pair of Visaton w130s-4 that I would like to make a budget build with, preferably an established design. I found several on the German Hobby HiFi site, the Boxschule, MiniSat, and Basic Monitor Mk2. So far I’m thinking the Basic Monitor Mk2 is my best bet, simplest crossover, vented for lower bass, and a bit larger. The tweeter on all these designs is the Visaton SC10N-8. These are available at Digikey for only $20 each, making total driver costs for an MT using these drivers around $100. I have pulled together information on the Basic Monitor Mk2 and will present it for comments/ideas.

When I was a young pup, I took an electronics course that culminated in the construction of a three tube amplifier. I think it had what was called a line voltage supply (no transformer) with potentially a hot chassis. Two of the tubes were 35w4 half-wave rectifier and the output was a 50c5. The the preamp was maybe a 12au7. The filaments were series wired off the line voltage. There must have been an output transformer but I don’t recall. I cut and bent the chassis on a metal brake and punched the socket holes. Using components from the tool crib and a schematic, I wired it together. I was hooked! I ended up making a nifty little intercom with it but it is very long gone.
I understand that this was a common basic project back in the day. Nostalgia has me wondering if anyone has any information on this little amplifier?

These boards were built five years ago, populated with Mouser supplied parts and assembled into an aluminum box, powered by a 24vdc adapter, included if required.
I never got into MC cartridges, and now offer it for sale.
$120 shipped.

Hey all,

I'm looking for a simple amp to run a pair of bookshelf speakers (Jeff Bagby's Continuum II) for TV use.

Criteria:

• We don't crank the volume very loud so it doesn't need to be high wattage, 30w will likely be more than enough
• Only need a single input, input is coming from the TV via RCA, so either RCA or 3.5mm would be fine
• Do not need or want Bluetooth or a remote
• Do not need tone controls
• Must be relatively compact
• Prefer an amp with low noise - can swap out the power supply if needed
• Must not make terrible noises when losing and regaining mains power (see below), or when plugging in
• Will be left on most of the time, so some sort of standby mode would be good
• Prefer a short wait time for waking up from standby, some amps I've been playing with have a 5-10 second lag (Dayton KAB/KABD amp boards)
• Subwoofer out (powered or not) is not required but might be nice to have
• Would consider an amp board and building my own enclosure
• Don't have a strong preference for amp style, ie: AB vs D, I imagine in my price range D will be my only option?
• Budget $250 or so

Currently, I am running these speakers in my office with a Fosi TB20A which is based on the TPA3116. This amp meets most of my needs except that when we lose and regain mains power, it makes a terrible looping, thumping sound until I power the amp off and back on again. My wife will murder me if I try to use this amp in the living room.

I've used the Dayton (Sure?) KAB/KABD amp boards in a couple of bluetooth projects and they're pretty neat, especially the DSP ones. However, they tend to be somewhat noisy and as I mentioned, the standby delay is pretty extreme - my wife will start a show and think the amp is powered off.

Any advice or thoughts are appreciated, thanks!

Hi. I was calling all Cambridge Audio users with the latest generation of receivers the AXR series. I just got the AXR85 and it sounds great. Very full bodied warm sound. I’ve owned a few of their integrated over the years and loved them despite two dying but they were over 15 yrs old at time etc. my issue is that when I use the balance co took which is in the menu section and not an actual knob (hate that part) when I turn balance all way to either left or right I still get sound out the opposite channel.

Got this Carver amp with C-1 preamp many years ago, probably 1989, second hand. The preamp was fine, but the power amp had some bad transistors. Worked after repair, but had a nasty turn-off pop through the speakers. I'd wince every time I turned it off. Like a full speaker excursion pop. Over the years, especially after about 1993, the two pieces sat in storage. No use at all until the early to mid-2000's, when I tested the stuff and let a friend borrow them for outdoor movie nights at his house. For whatever reason, the turn-off "pop" was gone, and it has been gone. My question is, what's recommended to do to these after so many years? I'm sure it needs re-capped and all the solder connections checked. But anything in particular? And wonder what that pop noise could have been?

Hello, have a nice day for everyone, i have a trouble with a tecnics SU-V800, the amp works normaly betwen a 4 and 5 minutes, after this time colapse end enter in of, the relay action are audible, but i measure the tensions on outputs and there is about 0,21 and 0,23 VDC, betwen chanells, more after 10 or more minutes the sounds retutn, and cycle repeats.
Regards, (i sorry by mi bad english)

Hi, planning to start a tube amp build next autumn, hardwired and as basic as it gets - just to get my feet wet.
I have built several projects before, mainly SS amps, most recent was a PassDiy F3.
Never a tube amp though. I’m aware of the high voltages.

Any recommandations? Plan is to drive boxes with either alpair or small Fostex.

Hi--

Looking for a pair of older or vintage-style 15" paper cone woofers. Smaller VC (1-2.5") ideal, like an old pair of alnico Jensens.

Goal is to xover around 1K.

Field coil or Alnico please (no ferrite).

Also not interested in big JBLs, Altec 416s or 515s--no problem with these, but I have some. Would be interested in earlier Altec 803As--the ones without the fabric accordion surround.

16ohm preferred.

Would look at newer stuff too, e.g., Supravox.

Let me know what you have.

I'm in the U.S.

Thanks!

Has anyone else seen the this? By jdflyback .
'buiding a 1920svtube amplifier (and tubes)'
Absolutely amazing.

Apologies if it has been discussed but couldn't find anything with a quick search.

This is a new thread for the Balanced Zen Line Stage Preamp. This discussion started at Which Preamp for Amp Camp Amp.

I’ve a mullard tube that starts E80??

The etched code is BG2 D1F4

Can anyone interpret this code to give me the type of valve - it has a pinched waist !

Hi. Hope you are all well.

My Technics SUV5 has an issue. I would be grateful if you can help. Relay will not come on for 5- 10 mins after switch on. Either a failing cap or transistor or cold solder. Resoldered relay but no change and some wires that had been previously replaced, When relay does come on it scrapes and struggles to click. Once running it is fine and relay switches on and off nicely with speaker selection. Voltage at relay is 50 v with relay off but once running 25v.

Best Wishes

Hi all,

inspired by the Nelson Pass' Zen v9 and Tubelab's UNSET, I woud like to propose you a schematic and ask some suggestions on how to choose the right components and/or working points. It is a single ended, single gain stage amp with triode curves, a DF of 2.4 and 26 Wrms at 4%THD.

The basic concept is that the upper n-Mosfet is driven from its source by the lower p-Mosfet source follower.
The gate of the upper n-Mosfet receives a feedback from its drain through a voltage divider to linearize it and triodize its curves: the higher the ratio, the trioder and more linear the curves and the lower Zout.



I attach here the schematic hoping to receive some help on the choice of the lower p-Mosfet with a reasonable SOA.
The CCS should be right as I've used the same power supply voltage and CCS circuit as the Zen v9.

It is supplied at 46 V B+ and biased at 2.6 A (1.3 A each IRFP240), being able to deliver (on simulations) 26 Wrms with the following THD:


And 1 Wrms with the following:


The DF is 2.4 on 8 Ohm (Zout is 3.3 Ohm) and I'm sure that it can be done way more than this with better working points and components.

Thank you in advance

Roberto

This is an attempt to provide some quick access to all functions of horn resp. Criticism is welcome.

I've got an active thread going about my Audire Andante preamp repair efforts
Need Your Assistance - Audire Andante
and was encouraged to share with the larger forum the following information:

My brother was an early client of Audire and Julius Siskinius, until the time of Julius' passing. At first, he owned various Audire amplifiers, with high-end components and speakers from other manufacturers. After trading up to better Audire products, Julius agreed to custom-build some very, very large full-range ribbon speakers for my brother.

His were the only example of the speakers ever made, and were originally fed with 2 Otez, but they were more efficient than expected, so Julius took back 1 of the amps. The Andante dual mono, full-feature preamp I'm trying to repair completed the set. The speakers basically consisted of an array of four ribbon sections from his then largest panel speakers. I've included a photo of the largest production speakers, and a photo of a single one of my brother's.

Julius had to make a special press table to assemble the frames and hundreds and hundreds of little magnets on either side of the mylar film panels, which he tuned for response by varying the doping along or across the mylar. Julius tuned these for mostly classical music, primarily piano as requested, and he had to got through a lot of trial and error to get the response right. Since each panel had to produce the full frequency spectrum, it's staggering to think of the time and effort this took. Jazz, of course, was also wonderfully presented. Listening to Miles Davis' Kind of Blue on these was a religious experience for me. I've listened to speakers costing up to $150,000, and have never heard anything like these. They became members of the family.

Julius later told him he would never make another of these, because it was too much work and he couldn't sell them to enough buyers for enough money.
After about 30 years when they needed repair, Julius was too old, and had gotten rid of the special press-table. When the foam tape between the frames and diaphragms started to decay, my brother thought he could just disassemble them and replace the foam, but the magnets, once released, instantly formed a huge pile of multi-segment magnet.

That Julius worked in the space program was never surprising, seeing the quality of the work on his gear and selection of components.

Cheers,

Greg

Greetings!

I traced out the crossover for the Tannoy CMS1201 celing speaker.




Please help me figure out how to shelve the tweeter by about 3 db without messing up.
Also, what is the function of the pair of 24 ohm resistors (they are 30 watts each)?
I see this is a 2nd order high and low pass crossover but the tweeter network is confusing me since this does not look like a zobel.

thanks!

Hi all,
following are different THD+N measurements of random two different brands of amplifier taken from website audio.pl. Being Distortion + noise measurements I am curious what makes one chart having THD+N higher than other at 0.1watt. At such low level what contributes more to higher level (shown in green circle) noise or distortion ? Both amps are (Emotiva and Denon) class AB and more or less same power





thanks and regards

I have gone through a lot of speakers over the years trying to find a good match for my Sansui AU-11000 amplifier. The best Ive found so far are a pair of rebuilt DMC QED 1A speakers, I'm trying out some Wharfdale Valdus 500 speakers at the moment, they don't sound too bad but I prefer the DMC speakers so far. Is anyone else getting good results with their Sansui AU-11000/AU-9900 on their particular speakers?

Hi

I built a self-biased 12AX7 cathode follower with the usual sort of values:
Rk bias resistor 1k5
RL cathode load resistor 56k
Rg grid-leak 470k tied to junction of Rk and RL

When I power it up from a 300V supply, there is about 70V at the tube cathode, about a volt and a half across Rk but the grid is at about 45V. I have built this using different sockets, different tubes, and different resistors and the voltages always come up weird.

If I short RL then the grid is at zero volts, cathode at 1V5. The current through the tube seems the same either way.

I thought maybe there was something going on during the warm-up and added a grid protection diode. This did not change anything.

With the voltages present I would expect the tube to be off, but it works and passes signals like it should. The DSO 100MHz scope shows no oscillations or noise.

How can this be? Does anyone have any ideas?

Thanks

I am trying to generate about 350V from 420V unregulated - about 50ma. The regulation for this design is not the best - I was wondering if it could be improved or other ideas welcome.

I've got no room in my life for oversized speakers anymore, so it's time to get rid of my Dallas II horns. Is the FE206E a useless driver for someone who doesn't want big complex speakers?

Hi all
I have an old car stereo (Volvo TD-603 1992) which I have refurbished by removing the cassette mechanism and replacing it with a bluetooth receiver. That all worked well, but that's not what I'm writing here about...

The stereo - which outputs 4 channels to an external amp - lost one of the channels - rear right (RR). I followed the traces to find that the RR channel had a broken trace near the output jack, going between that and a break-off PCB which has vol, bal and fader controls. I repaired this channel with some wire soldered in, but it's still doing some funny things:

May I ask these questions...
1 - The PCB has white traces printed on the top-side of it, and these appear to be actual working traces - is there such a thing as this? It's an early 1990s product. Are the traces carbon or something? One of them was scratched and broken, but there didn't seem to be a way to repair this trace, other than solder a lead onto the pins on the bottom-side which has more standard PCB traces and solder pads.
2 - Each of the four channels has a trace going from this vol/bal/tone/fader board, carrying the audio signal to the output jack connector. This trace was broken on the RR channel. But when I was measuring continuity on these traces, I found that the three channels which still worked all had around 30 ohms between the vol/bal/tone board and the output jack - yet there doesn't appear to be any components or resistors there which would add this 30ohms - the only thing I can think of is that these white traces have more resistance than standard copper.
3 - Is this 30ohm deliberate? And will it change the audio signal considerably for the RR channel which (due to the repair) has 0 ohm in the same place where the other channels have 30 ohms? Or should I put in a 30 ohm resistor to bring the RR channel into line with the others?

The funny things it is doing is: sometimes it sounds like it's mixing left and right channels together - there is a sort of phasing sound. I am wondering if the output channels having different resistances could be making it go slightly haywire? Just a thought.

Many thanks for looking at this, and any thoughts gratefully received.

I have some drivers to sell from the highly regarded & rare Polk SRT speaker system.

- 8 x MW5804 5.25" drivers - all in working condition - $45 USD each,

- 6 x MW5805 5.25" drivers - all in working condition - $45 USD each,

- 2 x SL-6501 trilaminate dome 1 inch poly dome tweeters - working well - $45 USD each,

- 2 sets of connector terminals and xovers for these speakers - $70 USD for all that.

Shipping would need to be made from NZ. One of the mid/woofers has a small 1 cm-long rip in the surround which will be glue repaired.

Shipping and postage would occur after the xmas madness, shipping will be from New Zealand.

Pictures can be posted here and/or DM'd - no problem.

I have been a bona fide member of this site for many years, and also at audiokarma.com (as 'slimecity").

Note that the numbers of the available drivers is subject to change - as I intend using 2 mid/woofers in another project - just not sure which ones will work best yet.

Please message me your interest. thanks

Details (please read!):
In the spirit of keeping things equally unfair to all, there will be a lottery to choose participants, drawing will take place April 10th.
If chosen, you will need to pay $750 by April 15, 2022 to keep your spot.
Spots that are unpaid after April 15 will be filled with other lottery participants, moving down the list so if you don’t initially get a spot, there is still hope.
Please only enter if you can attend in person. If you can’t make it to the camp for whatever reason, you will still be able to come collect the entire kit and take it home to build.
Sorry, no refunds.
Sorry, no shipping of kits. They must be taken away from the camp site after the build or picked up if for some reason you miss the build.
Car and tent camping at the site is fine the night before and the night following the camp. There is one bathroom out in the barn available for freshening up, otherwise try to be as boondocking-capable as you can.
Bring family and friends to observe or participate as appropriate. Any children in attendance will need supervision away from the build area.
The Moth full range driver is in a relatively small open baffle.
The downward facing Eminence bass driver is slot-loaded.
For best listening the speakers should be placed so the baffle is at least three feet away from the wall (so 18" from the rear of the bass driver to the wall).

http://nichicon-us.com/english/products/pdf/e-vz.pdf
I couldn't find the ESR for this series, they say it has high ripple.

I chose this series because its the only one that covers my whole amp
without taking caps from a different series or brand.

It's for my amp (all caps except signal path and ceramics).

I wanted the Elna Silmic II series but it doesn't have a 330uf 63v
and 0.22uf 10v caps and i don't want to have my whole amp with
one series except for two caps from another brand.

What do you recommend? does it really matter if it's not in the signal path?
Does the Nichicon VZ series has low ESR?

Not sure if this has been posted before, but I've never seen it.
Guy making a 1920's style amp from scratch, tubes and all.
Two DHTs.
Brilliant.

Building a 1920s Tube Amplifier (And Tubes!)

This issue is not with the notorious startup and shut down pop on this receiver, but rather a popping sound on start up. It lasts for about 2 minutes and then goes away I presume a warm up. It happens in both speakers even with the volume at 0. I am thinking power supply. I did the old freeze spray trick on the TR903/2SC932 bipolar transistor. I am fairly sure that this is the culprit. After a spray it stopped the popping. It maybe on it's way out. Any suggestions for a replacement" I have found this a BD241A/C and need confirmation as to it's compatibility from the more knowledgeable. http://www.mouser.com/ds/2/389/CD00000938-14108.pdf

FYI most of the lytics have been replaced on the pre driver, power supply and preamp boards.

Schematic attached.

Optimus Pro Lx5ii replacement crossover. Is there a direct replacement I can just buy to replace it with better components

Everytime I turn on/off my vintage Harman Kardon 930 receiver, my woofers on my speakers "pushes out and then gets sucked back in." Almost violently.

I have tried different speakers with the same results.

Does anyone know what likely could be the problem?

- Sushimaster

Fern & Roby is a small company that makes idiosyncratic loudspeakers, and also two turntables. The pictured one asks $8,500; the one with a 30-pound solid brass platter asks $10,500. Both have Richlite plinths. The stuff (Richlite) is dead as all get-out, and at that price tier, one assumes that Fern & Roby could have chosen almost any material.

https://www.fernandroby.com/products/details/the-montrose-heirloom-turntable-1

I continue trying to work the angles to get a pair of demo loudspeakers built, one in MDF, the other one identical except for the cabinet being made out of Richlite; and I will post any developments as they occur.

amb,

john

The 930 is known for a loud pop after turn on, however, the particular unit on my bench keeps on popping for a few minutes after.

I first pulled the Main-In/Pre-Out jumpers and fed a signal directly into the amp. Other than the typical HK initial pop, there were no further eruptions. So the amp section would seem to be ok.

With the function switch set to Aux 1, I measured the voltage at the output (R) of the Tone Control PCB while also measuring it at the speaker output (R). I chose Aux 1 because this bypasses the Preamp board, making the Tone Control board the only board between the RCA input and the Preamp Out. Here's what I measured:

Initial Turn On

Tone Control: quickly fluctuating voltage as high as 1.4V DC.
Speaker: quickly fluctuating voltage as high as 19V DC.

First Three Minutes
Tone Control: voltage settling down to between 600mV and 700mV
Speaker: voltage still going as high as 2 - 3V, but beginning to drop to 150mV in tandem with the output of the Tone Control board.

After 10 Minutes
Tone Control: voltage fairly steady at 465mV
Speaker: voltage staying at around 115mV (still high for DC offset, so I'll likely replace the differential pairs to see if I can lower it)

Power Down
Speaker: spike of 16V or so, gradually dissipating.

I'm thinking there's either a leaky cap or a bad transistor on the Tone Control board. Maybe more likely a transistor since the issue seems to disappear after warm up. Unfortunately, the service manual doesn't include any reference voltages to measure against, which is how I would usually track down the offending component. While I can hit each transistor with a quick shot of freeze spray, I'd like to hear how others would suggest narrowing down the possibilities.

Hello all,

This is a tripath TA0104A driver module from a Zapco car audio amplifier that is going into protect. I've got a thread going on this amp in the car audio forum, but have not had any response on what is hiding under the black blobs, so I thought I would try here.

Can someone tell me what the ICs/components are under the epoxy? If I could find a data sheet I might be able to surmise what they are doing.

What is the component labeled PLANE in between the two epoxy blobs on bottom of the picture?

Does anybody know of any method to test the TA0104A to tell if it is bad? After reading the other posts about these modules I realize they have long been extinct so If I could determine if it was bad that would give me an idea if I should be spending any more time on this amp.

Another thing, this amplifier has an ATmega103 M.C. which is also unavailable so I need to determine if it is good or not , if anyone has any advice on testing that.

Hi I am near,new paltz,Woodstock,Kingston,rhienbeck,etc
I am doing a big move and very busy with the business and helping my wife's business so limited time to diagnose
Fix a aca camp amp ,one does not work (brought used from this site)
Fix a kenwood ka 3500 ,one channel oscillating
Few other projects also depending on your time and $
Thanks in advance

Here is one question about isolation transformers in use for audio systems.
Here comes to the shop one machine which is one isolation transformer but its configured in a way that i am not familiar with

Transformer is torodial
The primary has a tap for 230 -240 use ...fine with that
Secondary though is made from two windings of 115 V center taped .
so secondary will provide side to side 220-230V depending in the input
The center tap though goes to ground connected also in the enclosure and to the mains ground

I am not familiar with this practice can any one comment on pros and cons ?
kindest regards
Sakis

I have already proposed a number of unhinged chip-amp projects:

https://www.diyaudio.com/community/threads/automatic-resonance-frequency-finder.254384/post-3886008
http://www.diyaudio.com/forums/chip-amps/176052-now-regulator-chip-jlh-amp.html
http://www.diyaudio.com/forums/chip-amps/192934-se-class-regulator-chip-amp-madness.html
http://www.diyaudio.com/forums/chip...oss-tringlotron-regulator-chip-amplifier.html
http://www.diyaudio.com/forums/chip-amps/193214-class-chip-amp-now-complementary-version.html
http://www.diyaudio.com/forums/chip...ator-chip-amp-family-welcomes-new-member.html
http://www.diyaudio.com/forums/chip-amps/175457-just-fun-regulator-chip-amplifier.html
https://www.diyaudio.com/community/threads/yet-another-funny-chip-amp.280777/post-4476559

Here is another one to add to the family: it is a solar "helper". It has the ability to inject a modest amount of power into the mains, from an array of photovoltaic cells.
It is not a solar inverter: it has no oscillator or timebase, no synchronization circuit, and it is incapable of operating as a stand-alone power source.

But how can it work then?
That's where the chip-amp steps into play: it is configured as a power negative resistance, connected to the mains via a transformer, to adapt the voltage, and for obvious safety reasons:



The circuit is based on a TDA1512, just because I had one patiently waiting in a drawer: any other chip-amp or power opamp would be suitable (and probably preferable, because the TDA1512 has annoying quirks).

The load (the transformer's primary) is connected between the output and the - input, and the current is sensed by a low-value resistor (R8, R9). On the + side, a resistive divider (R3, R5) injects a positive feedback signal matching the voltage on the sensing shunt, to generate the negative resistance.

The negative resistance engine itself is minimally simple, but there are lots of auxiliary circuits around: they are needed for a smooth, effective operation. The project is small-scale, but fully fledged and is much more than a mere proof of concept: it has to handle all the possible situations, and behave gracefully and safely all of the time. This requires optocouplers, relay, etc.
It is a lot of circuitry for a meagre 10W (peak!) output, and a continuous operation for more than 10 or 20 years would be required to amortize the building costs (unfortunately this might be reduced to two or three years with the rocketing energy prices), but it is fun, and it could be upscaled relatively easily.

In the subsequent posts, I shall describe in detail each section of the circuit, but in the meantime here are a few pics of the build:





And this is the indoor unit:



To be continued.....

What drive units are theese?
Is it vintage Jordan or maybe Bandor units? I tried to Google but no luck.

Hello everyone! I recently made a pair of small nearfield speakers using a modified Ciare PA065 fullrange driver. I love how they sound, but they can hit about 70/80Hz in my room, under that there isn't anything and the cone moves a lot. With certain songs it's completely fine but some others make me wish I had a subwoofer. I have some spare speakers I can use to make a "bass reinforcement" sub for this application, none of them is particularly worth of notice but they all should work ok. I want to spend as little money as possible for this project.

I can either use a 5" Faital 2+2ohm woofer I ripped from a Fiat subwoofer or a 8" BESTONE 4 ohm woofer. The latter one seems particulary well built with a very big magnet but I haven't measured its parameters yet. Both were in ported enclosures, but while I highly doubt the Faital will be able to produce any bass in a sealed box, the Bestone should be able to move some air. I measured the Faital even though I don't have the parameters on hand and that is also pretty decent.

I would prefer to use the smaller Faital woofer which will require a bass reflex enclosure, but if BR can't be used in this placement I can use the other speaker. I can EQ the system as much as I want to because I will only use it with my PC. I don't have any TSP now but eventually I will measure them later.

The crossover point will probably be around 100/150Hz because I don't want to spend any money on a dedicated amplifier with LPF/HPF so I'll just rip the board out some 2.1 PC system.

I want to place the subwoofer either like this:
or like this:
the dimensions are (all in centimeters)
a=34
b=100
c=10
d=10
e=15

If I want to use the 8" I will have to mount it with the magnet outside.
Do you think this can work?

For sale: New Lavoce SAN215.30 driver in original box.

Located in The Netherlands, shipping is possible at buyers expense and risk.

Price: 450 euros.

Ich habe einen Mark Levinson Ml23 Verstärker und habe folgendes Problem! Ich kann den Ruhestrom auf dem linken Kanal nicht einstellen! Poti ist
ok ! Ruhestrom ca. 13mV und die Kühlrippen sind eher kühl. Am rechten Kanal habe ich 11mV eingestellt und ist lauwarm.
DANKE für die Hilfe!
Andreas

Pls has anyone ran into a problem with Tpa3118 mono amp? I used 2pcs in a stereo configurations, but at a time, I noticed that one side of the speaker cutting off and on when you turn the volume up upto 70%. But right now, it has stopped completely and you will hearing something shooting like pa pa pa.

Hi everybody,

I am offering to sell my Audiosector NOS SPDIF Premium DAC. The unit assembled from the Peter Daniel premium kit is powered by the custom winded toroidal transformer (EU 240V) and it all sits in the nice wooden box with the elox aluminium front panel. I was using it for about one-year time (back in 2008) and since then it sleeps in my drawer as I don’t use my CD transport anymore…

The price is EUR250 - about of what I paid for the parts (DAC kit, transformer, connectors,…). Shipping cost would be approx. EUR15 within Europe and EUR25 outside world.

Anyone interested?

Thanks. Filip

Hi.

I get what looks like a "resonance phenomenon" when doing near field measurements on my speakers (both speakers). The wrinkle on the graph is visible around 48Hz, both when I measure the port and the speaker element. When I block the port, the wrinkle on the graph is still visible, but it's not as pronounced. See attached pictures.

The baffle is 22mm thick and the edges of the holes are not chamfered.

Does anyone have an idea what could be causing this, which I believe is a resonance?

All ideas are welcome!

Dear forum,

I'm trying to build a push-pull amplifier and I am struggling to find an acceptable way to wire the 12V to the tube connectors. I've enclosed a schematic representation of the lay-out of my amp. I am afraid the filament wiring will interfere with the input wiring.
I would really appreciate some input on how best to achieve my wiring.

Full recap of the power supply and pre driver board. All new replacements for
TR603, TR606, TR605, TR625, TR626 and TR623 on the pre driver board. This has solved my popping issue that I posted in another post. Bit of a hand grenade to be sure.

My issue now is that the bias on the 2 variable resistors on the pre drive board has very little effect on the erratic behaviour of the DC voltage. The amp actually sound just fine. No distortion, not loss of signal. I am thinking that the problem is on the pre driver board. I don't think is the power supply. Perhaps the remaining transistors need to be yank.

Please jump in if any of you knowledgeable guys got a fix.

I have just got a Tek 2225 oscilloscope, but this will not be up and running until I get some recos on a signal generator. That's for another post🙂

Thanks.

SCHEMATIC HERE!
Harman Kardon 930 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine

They never stop making tapes.

https://www.smithsonianmag.com/trav...-they-are-flying-off-factory-floor-180979417/

https://www.davehoekstra.com/2022/04/06/the-camelot-of-cassettes/

There is anywhere a broken tooth from a gear for tape transport (between reel motor and reel gear for cassette transport - go to page 43 service manual)). This kind of relacement parts are no longer available. Therefore I am looking for the same model in not working condition.
But above mentioned model is too rare.
Therefore I want to know, which other models from SONY uses the same cassette transport mechanism.

Maybe one of the member know this.
Thank you very much for advices.

Serv.-man. of this model is here:
SONY TC-K770ES SM Service Manual download, schematics, eeprom, repair info for electronics experts

(Almost) finished project. Digital 7.1 audio decoder for DTS, AC3, AAC, and all that FFmpeg will decode and fits on a TOSLINK bandwidth.











Sources and design (all GPL v2):

• Hacked Davinci Audio Driver
• Alsa driver and decoder software
• Eagle design files (circuit and board)

Project page, to be expanded

I have a ton of parts in stock from Cardas and Furutech. I have a lot of the Cardas panel mount XLRs available (male and female), multiple sets of there different posts, about 3000 ft of assorted Cardas wiring, 10 male and female XLR terminations (can send new in packaging or terminate with the wiring of your choice, a fair amount of 10 and 15a Furutech filtered IEC inlets, a few different sets of their short posts (amp side) and probably a bunch of other things I am leaving out.

I have attached a pic of the terminated Cardas XLR with a (diamondback) wrap and have about 10 different wraps in to pick from if you would like some terminated wires. That wire is a cryo treated OFC stranded cable wrapped in cotton but I also have solid silver Neotech wiring in stock if you prefer as well as Cardas.

The pricing is stellar on all parts and all are new and in factory packaging. If you are interested in any parts please send me a PM about what parts you would like and the quantities and I will reply back as soon as possible. All of these are available for immediate shipping.

I saw that most phono tube preamplifiers are posted in the Tube/valve amplifier section when they actually belong more to Analogue source, but from what I saw the analogue source is usually deemed to cover solid state preamps or hybrid aproaches at most.
Could it be a very clean choice of Tube preamplifiers section?

Hey folks, I hope you are all doing well. I know it's been a long time since I've stopped in. Life has been incredibly busy.

I what I hope will be a quick question, but the past is lengthy and I apologize in advance.

I have a PG elite.5 that a client sent me. When I got it, capacitor C515A had exploded in blaze of glory. I cleaned up the board, replaced the cap, tested all the transistors and their drivers and they all checked out fine. I powered it up and it draws about 3.8a on a current limited supply and input voltage sags to about 8.5V, I'd normally not expect an amp to even power on with the supply voltage dragged down that far, but it does. All channels come up and produce audio. If I start it on a unlimited supply, it draws about 2.5 amps and all channels produce audio.

Here is the weird thing though. And I've never seen an amp do this. If I power it on a current limited supply, and then while it's running, switch it to an unlimited supply, it begins drawing about 12.5A. If I start it on an unlimited supply and briefly cause the input voltage to sag, it starts drawing 12.5a and while in the sink it will eventually thermal.

Things I've found while troubleshooting:

1) With the rectifiers removed from the class d power supply, it doesn't exhibit any of the above issues.
2) All of the output filter caps as well as the final inductor have been removed and tested fine
3) When it's started with a current limit, the class d section oscillates at 67kHz instead of 45kHz. I don't recall for certain, but I believe when I make it exhibit the issue with input voltage dash that it also starts oscillating at 67kHz. I can verify if it's relevant.
4) when started with a current limit, the low side drive supply only produces 6V above VNN, not 12V.
5) PWM drive for the power supply looks completely normal with no shot through under all test conditions, however the drive is roughly half the amplitude when it's having issues. I suspect that's just a factor of the voltage sag from the current limit, but I'm not 100% sure. The PWM seems to be locked at 49% duty cycle regardless of my test conditions, but it's possible I've never raised the supply voltage high enough to get the PWM to roll back duty cycle.

Things I've tried to do:
1) I replaced the class d output fets. The schematic calls for IRF640 and it had been previously repaired with mismatch IRF640Ns.
2) I replaced r class d driver bjts
3) I replaced the two mmbta42 low side driver bjts on the driver card.

I'm really stumped on this thing. I don't do a lot of class d as it is, but I normally can hold my own. I still have no idea what caused the capacitor to explode in the first place. I feel like it shouldn't be powering up at all with the voltage so low. I have schematics for similar designs and they all have undervoltage protection on the PWM board. It appears according to the aamp schematic, that this one does not, however the schematic I have and the amp have several noticable differences.

The tl;dr is as follows:
On a current limited supply, the amp runs fine and draws about 3.8a. On an unlimited supply it runs fine and draws about 2.5a. If it starts on current limited and is switched to unlimited while it's running, it starts drawing 12.5a. If it's started on an unlimited supply and the voltage sags, even very briefly, it starts drawing 12.5a.

Thanks in advance folks!
Jason

I aquired this amplifier. I have very little education and experience with electronics but trying to understand. I have not seen an amplifier design like this before. It is made by Foundation Research in Canada. I can not find info on it. The transformers are Hammond. Has a total of two Transistors 2N5684
I don't think It would be an easy sell so I'd like to use it for whatever it would be best at.
I have a few questions
1) Could you point me to a lesson on this type of amplifier?
2) Is this a good quality amplifier?
3) Does this design have any specific strengths ( clarity, decibels, bandwidth ) or weaknesses.
Thanks

IRON SALE Audio In Int Power OT transformers Chokes
I have a lot of iron I've gathered up over the years and need to free up some space. All windings have been tested for continuity and chokes have been tested for inductance and DCR.

Audio Input And Interstage:
No Name T101 potted cylindrical. 200 ohm to 500 ohm or visa versa. 1 1/4" mounting, 2 1/8" tall. $15
ADC A7185 Input transformer $20
Thermador Audio Interstage 3 windings, 2 center tapped. Large 3 5/8" x 3 3/8" x 4 1/2" $25

Audio Output Transformers:
Organ amp with power and output for quad of 6BQ5 to 4 ohms or run a pair at 8 ohms. Tested before removed and sounded great. $50
1956 Lowrey organ 6V6 output transformer. PP to 8 ohms. Sounded great before removed. $25
Triad S-31 output transformer. 8000 PP CT to 4,6,8 ohms. $50

Power Supply Transformers:
Pair Triad R112A. 550V 110mA CT, 6.3V 5A, 5V 2A. One has stubby leads. All windings good. $100/pr. SOLD
Triad R9A. 600V Ct 75mA, 6.3V 1A, 5V 2A. $25
HP Paeco 9100-0050. 840VCT 70mA, 6.3V 1A, 6.3V 2.75A, 7.3V 600mA 3 1/4" x 3 3/4" x 4 1/8" $25
Freed 800V CT 100mA, 6.3V 1.25A, 6.3V 2.5A, 5V 2A. 3 3/4" x 4 1/4" x 4 3/4". $40 SOLD

Chokes:
Coil Co of America 5.2H 60 Ohm 3 1/4" x 2 1/2" x 3 1/2" $10
Freed 6.6H 233 Ohm. 2 1/8' x 2 1/2" x 2 3/4". $10 sold
Sherhold 2.8H 80 ohm. Cylindrical 2 1/2" x 3 1/4". $10
UTC? Navy seal. 6H 90 ohm. 4 1/2" x 3" x 3 1/2". $30 SOLD
Stanley 2H 22 ohm 700mA. 3" x 3 1/4" x 5". $15.
Midwest Radio 6H 95 ohms. 2 5/8" x 2 5/8" x 3 1/2". $20
General Radio Inductor675-31. 5H 72 ohms. 2 3/8" x 2 7/8" x 4 1/4". $20
+ Shipping

Buy a bunch and stuff in a Flat Rate box for extra savings!
pp f&f zelle please

A good friend in Germany is a loud speaker builder. He has recently been totally hooked on active crossovers using the popular Mini-DSP. The digital solution is of course very convenient. One can just change the crossover characteristics on a laptop, check the total response in software based on measured chasses frequency responses, and then write it to the DSP. And we can immediately listen to the result / changes within minutes. A direct A-B test of two different crossover designs becomes possible.

But once settled on a final design, we wish to convince him that analogue filters sound better.
John Curl's Blowtorch preamplifier part II - Page 9870 - diyAudio
John Curl's Blowtorch preamplifier part II - Page 9870 - diyAudio

We were looking for a modular active crossover filter system where we can combine different basic filter elements as described by Linkwitz, for example.
Active Filters
In particular, we do not wish to use DIP switches in the signal path, and we wish to be able to vary the number of elements in series as we wish. After searching the net for solutions / examples, we found nothing suitable, and ended up designing our own.

You can read about our solution in the attached PDF article.


Happy reading,
Patrick

.

Sadly after a tussle with DPD on sending this out to the previous buyer the Hypex unit failed. It has been returned to Hypex, serviced and is in full working order. As a result of the last experience I am listing this for collection.

You are quite welcome to arrange your own courier to pick up but you will be responsible if anything is to happen to the items, I will not accept returns.

I am also leaving the units as they are. I won't be fitting the unit that has come back from Hypex unless you require me to but if you can use an allen key and a cross head screwdriver it's very simple. I've included photos of the case and the Hypex amp and also previous photos of the unit fitted. I will also be including a power cable and a custom DIN to XLR lead designed for use with Naim pre amps. This makes an exceptional combination and easily punches above the Naim NAP 200 and NAP 250 in terms of performance for a fraction of the price.

Collection: Oxfordshire UK
Price: £550

Reasonable offers considered

FEATURES:
Two channel amplifier
5W standby SMPS
Advanced over current protection
External controlled operation
Auto-switching (115/230V)
Low weight: 1055g.
Compact: 230x115x47mm

HIGHLIGHTS:
High efficiency
Universal mains operation
Flat, fully load-independent frequency response
Low output impedance
Very low, frequency-independent THD
Very low noise

nCore Specs:
Power (2Ω) 2X 450W
Power (4Ω) 2x 500W
Power (8Ω) 2x 350W
Channels 2
Weight 1055g
Dimensions mm 230*115*47
Mains voltage input 230V / 115V
Automatic input voltage selection Yes

Ghent Case:
193mm(W) x 58mm(H) x 315mm(D)
Color: Full Black
Face-Plate: 8mm thickness & NCORE logo
Top-Cover: 3mm thickness, with standard venting holes
Bottom-Plate: 3mm thickness, pre-drilled holes for Hypex NC502MP (Free drilling for other modules)
Rear-Plate: 3mm thickness, one pair of XLR sockets, two pairs of Binding Posts, IEC socket with power button
DC Supply for LED Dimmer: DC +10v ~~~ +26v

selling one pair of very rare TANGO FG50S for hiraga Nemsis amplifier or any SE mono transistor project . comes with a pair of genuine 2SK135.

Asking 1000€ shipping oversea is possible but will be expensive.

I have 2 pairs (4pcs) Tokin 2SK180 for sale. Bought from another member here.
They are unsoldered.

Asking for $75/pc + shipping.

In the past, I built a range of fairly simple (speaker) amplifiers using ready-made PCBs, which were successful and quite fun. Prompted by a kind PM from another member, I'm toying with the idea of trying something just a little bit more sophisticated next, with particular interest in low noise design (for use as power amps for close and potentially sensitive speakers). Such as having good power supply rejection ratio and low noise input stage etc.

I've some idea of features this might entail, but to increase chances of success I'm really looking for proven designs to either make or at least start from. I've decided that class AB would suit me best, but there are still a bewildering array of possibilities on diyAudio. So I wondered if people could suggest some good low-noise designs that I might look at trying to build?

Thanks,
Kev

On the bench is an Lafayette LR-9090 that I'm doing a little resto mod on to stabilize its power amp and reduce the power amp's distortion performance to >100dB.

Can we upgrade its preamp to the same level of distortion performance?

The -9090 preamp has 3 gain stages. In spice, two of them already perform at the same 100dB-or-better distortion level. Awesome.

The third gain stage is a simple emitter follower buffer that drives the high and low filters; it's in the signal path regardless of whether the filters are engaged. This stage is only good for about -80dB distortion performance. The distortion is caused by a combination of nonlinear Cob, nonlinear Early effect, and nonlinear Vbe variation. It sees an input impedance near 10K ohms, so any input current error caused by the nonlinear Cob or Early effect is multiplied by 10K. That's fairly stiff, but typical for some preamps.

I suspect we could replace an EF stage's transistor with the "transistor equivalent" circuit below to cancel all 3 distortion sources. The beauty of this is it's a 3-terminal circuit -- you could build it on a tiny piece of perf board and it's a "drop in" replacement for the original EF transistor for a minimally invasive, completely reversible mod. Spice shows it reduces distortion in this stage somewhere beyond -110dB.

It's essentially a normal CFP stage, except that Q1 is cascoded by JFET J1. The CFP arrangement ensures Q1 conducts constant current, so its Vbe is constant. The cascode nulls out any Early effect or nonlinear Cob at Q1. C6 is necessary for HF stability.

You could probably extend this with a voltage divider to create a very clean 3-transistor CFA gain stage.

Disclaimers:
1- I haven't built this -- yet. Gotta pick up some JFETs.
2- TBD what JFET to choose for a real build.
3- This circuit requires several volts more headroom at the collector than a single BJT. That's usually not a problem in a vintage preamp with rails in the 18V to 25V range.
4- There's more than one way to compensate this for HF stability. The one shown might not be optimal.

hello everybody
i have a pair of audiocontrol eqt mono eqs.
after having noise problems i narrowed it down and found that one of them is the cause of the noise.
can anybody guide me what is the problem with it?
thanks