I had a bit of time to play; connected my ACA's, dual monoblocks, power supplies installed inside the chassis, configured for bridged operation. There's simply no comparison, ACA's blew my (modified!) 820BX4 out of the water.
Incredible difference.
It's not that 820BX is bad, it's just that ACA's sound is sooo much more natural, fluid, extremely fast... and... spacious.
Build ACA's... and then move to Aleph J... or simply, go straight to Aleph J.
Incredible difference.
It's not that 820BX is bad, it's just that ACA's sound is sooo much more natural, fluid, extremely fast... and... spacious.
Build ACA's... and then move to Aleph J... or simply, go straight to Aleph J.
Attachments
Those twisted copper flat wires, your ACAs look stunning! Right now such kits are beyond my budget. My whole rig is worth $600. Good thing is it sounds quite exceptional for the money 
I removed decoupling 0.1uF MKS caps and put something more beefy in their place. For negative rail decoupling a 10uF tantalum and a 100uF electrolytic for the positive rail. I had 220R resistors in stock so I used them for the positive rail filter. I lost 2 volts DC there.
Now both rails have around 5mV of ripple. I can see a light drop in the 100Hz noise but the THD is more or less the same as before. Can't say if it's my measurement setup limitation or a valid measurement. Sound wise, I think the amp improved slightly.
I have a question regarding tantalum caps. During power on and normal operation there is no positive voltage on negative leg, although when I turn off the amp the voltage slowly drops from -18 to +2 and then to 0. I know tantalum caps are sensitive. It's a 10uF 35V part. Should I worry about it?
I removed decoupling 0.1uF MKS caps and put something more beefy in their place. For negative rail decoupling a 10uF tantalum and a 100uF electrolytic for the positive rail. I had 220R resistors in stock so I used them for the positive rail filter. I lost 2 volts DC there.
Now both rails have around 5mV of ripple. I can see a light drop in the 100Hz noise but the THD is more or less the same as before. Can't say if it's my measurement setup limitation or a valid measurement. Sound wise, I think the amp improved slightly.
I have a question regarding tantalum caps. During power on and normal operation there is no positive voltage on negative leg, although when I turn off the amp the voltage slowly drops from -18 to +2 and then to 0. I know tantalum caps are sensitive. It's a 10uF 35V part. Should I worry about it?
Last edited:
I wouldn't worry too much about what happens when you turn off the amp. I know tantalums are tricky... I don't use them any more. The fact they get exposed to a small reverse voltage, for a brief moment every time you turn off the amp, should not damage them.
Suggestion: the places where 0.1uF caps are used originally (local decoupling) on the PCB, should have a max of 1uF (if you really want to use "big" capacitance in this place), in combination with something like 0.01uF cap. You want a very fast rail, that has a a very low output impedance across wide spectrum range - into kHz range as well. Putting a 100uF (anything bigger than 1uF) there is not a good idea.
Suggestion: the places where 0.1uF caps are used originally (local decoupling) on the PCB, should have a max of 1uF (if you really want to use "big" capacitance in this place), in combination with something like 0.01uF cap. You want a very fast rail, that has a a very low output impedance across wide spectrum range - into kHz range as well. Putting a 100uF (anything bigger than 1uF) there is not a good idea.
Re: wire...in general... try using a solid core copper wire. For input signals, use a bare (unshielded) twisted pair, CAT5e solid core copper. Do not use litz... some CAT5e cables use litz twisted pairs.
For power and speakers, use thicker solid core copper wires. Again, avoid litz cables...
If you want to hear a real improvement in sound quality, use the TAPE IN input, cut the PCB tracks from RCA's to input switch, ground the tracks that you cut-out (do not leave them floating), and use the CAT5e twisted pair instead... and let me know what you think
For power and speakers, use thicker solid core copper wires. Again, avoid litz cables...
If you want to hear a real improvement in sound quality, use the TAPE IN input, cut the PCB tracks from RCA's to input switch, ground the tracks that you cut-out (do not leave them floating), and use the CAT5e twisted pair instead... and let me know what you think
Hi Extreme_Boky,
I have never seen any Litz wire used in CAT-anything wire. It would be too expensive to get the same equivalent gauge of wire with zero benefit (otherwise it would be all you would see). I've installed more CAT-3/5/5e/6 than I care to remember.
CAT-whatever wire does very poorly as speaker wire. Been there, done that again and again. Results always the same. The best is a fine stranded zip cord, or it can have space in between (doesn't matter). For really good wire inside, use something along the lines of RG-194. If you have radiating sources, run in copper tubing through the area. If everything is pretty quiet electrically, you can use straight runs of stranded wire. Solid wire tends to break over time. But, whatever makes you happy ...
You know what CAT-5/5e/6 wire is really great for? Network connections (imagine that) and for telephone digital and analogue sets. Cuts down radio pickup.
-Chris
I have never seen any Litz wire used in CAT-anything wire. It would be too expensive to get the same equivalent gauge of wire with zero benefit (otherwise it would be all you would see). I've installed more CAT-3/5/5e/6 than I care to remember.
CAT-whatever wire does very poorly as speaker wire. Been there, done that again and again. Results always the same. The best is a fine stranded zip cord, or it can have space in between (doesn't matter). For really good wire inside, use something along the lines of RG-194. If you have radiating sources, run in copper tubing through the area. If everything is pretty quiet electrically, you can use straight runs of stranded wire. Solid wire tends to break over time. But, whatever makes you happy ...
You know what CAT-5/5e/6 wire is really great for? Network connections (imagine that) and for telephone digital and analogue sets. Cuts down radio pickup.
-Chris
Did you check what I was writing before??
I provided a cheap solution to pure annealed silver/copper ribbons that I use, to a fellow member who mentioned he was on a budget.
I did not say use the CAT5e for speaker cable!
I have CAT5e cable made with litz strands; CAT5 do come in litz.
I followed your advise and got rid of 1Ks, put 47R in. Temps are great at the moment. Hot resistors are gone but also Q623/625 got much cooler.
Sound wise, I'm not entirely sure, I will have to switch back and forth with this mod and bias the amp exactly the same, although I feel like the sound got a little more bright and perhaps more brittle. Especially on high volume levels.
There is a really nice paper covering the principals of this topology. https://linearaudio.nl/sites/linearaudio.net/files/otala low tim amp.pdf
"Transistors T12 and T15 work in class A. When the output transistors are driven in class B, and are switched off alternately, T12 and T15 remain conducting, so that the charge in the base-emitter junctions of the output transistors can easily flow away through the low output impedance of T12 and T15. As a result the switching behaviour of the output transistors is very good."
T12 and T12 are Rotel's Q623/625. Obviously, the 47R mod takes the class A strain off those transistors. I'm not sure if my amp has ever kicked into class B. But class A bias for anything always makes sound better, right?
I put Solen MKP PPE220s and it worked out very well. The bass is now deeper than ever which makes me a little confused, because measured frequency response didn't change a bit..
By the way, I don't think red Audiophyler caps are made by Mundorf. They are most likely Chinese and quite affordable. At least in my country.
I followed your advise and got rid of 1Ks, put 47R in. Temps are great at the moment. Hot resistors are gone but also Q623/625 got much cooler.
Sound wise, I'm not entirely sure, I will have to switch back and forth with this mod and bias the amp exactly the same, although I feel like the sound got a little more bright and perhaps more brittle. Especially on high volume levels.
There is a really nice paper covering the principals of this topology. https://linearaudio.nl/sites/linearaudio.net/files/otala low tim amp.pdf
"Transistors T12 and T15 work in class A. When the output transistors are driven in class B, and are switched off alternately, T12 and T15 remain conducting, so that the charge in the base-emitter junctions of the output transistors can easily flow away through the low output impedance of T12 and T15. As a result the switching behaviour of the output transistors is very good."
T12 and T12 are Rotel's Q623/625. Obviously, the 47R mod takes the class A strain off those transistors. I'm not sure if my amp has ever kicked into class B. But class A bias for anything always makes sound better, right?
I put Solen MKP PPE220s and it worked out very well. The bass is now deeper than ever which makes me a little confused, because measured frequency response didn't change a bit..
By the way, I don't think red Audiophyler caps are made by Mundorf. They are most likely Chinese and quite affordable. At least in my country.
Douglas Self wrote a fair few articles on biasing AB amplifier topology into class A... have a look/read, if you haven't already...
Nice to hear you like the mod No 17 from my list
Hi Extreme_Boky,
I skimmed over previous posts and this one from you caught my attention. Besides, you wrote:
Anyway, you have me curious now. Where have you seen Cat-whatever Litz wire?
-Chris
I skimmed over previous posts and this one from you caught my attention. Besides, you wrote:
and I was merely replying in general terms as well to match your post. You did say use solid core wire after all, then started talking about CAT series of cables.
Stranded wire yes, but Litz? No. I have never ever seen it in any catalogue and as I said, it would be pretty pointless to use. The expense would be enormous! Can you imagine a 330 meter (1,000 ft) roll of that stuff!!! It is more important that the strands remain twisted at a constant rate and do not bend through each other. CAT-6 has a central core of plastic to make sure that doesn't happen.
Anyway, you have me curious now. Where have you seen Cat-whatever Litz wire?
-Chris
I followed your advise and got rid of 1Ks, put 47R in. Temps are great at the moment. Hot resistors are gone but also Q623/625 got much cooler.
Sound wise, I'm not entirely sure, I will have to switch back and forth with this mod and bias the amp exactly the same, although I feel like the sound got a little more bright and perhaps more brittle. Especially on high volume levels.
There is a really nice paper covering the principals of this topology. https://linearaudio.nl/sites/linearaudio.net/files/otala low tim amp.pdf
"Transistors T12 and T15 work in class A. When the output transistors are driven in class B, and are switched off alternately, T12 and T15 remain conducting, so that the charge in the base-emitter junctions of the output transistors can easily flow away through the low output impedance of T12 and T15. As a result the switching behaviour of the output transistors is very good."
T12 and T12 are Rotel's Q623/625. Obviously, the 47R mod takes the class A strain off those transistors. I'm not sure if my amp has ever kicked into class B. But class A bias for anything always makes sound better, right?
I must admit there really is something about this class A bias of Q623/625 transistors. A reversed the 47R mod and distortion jumped up slightly from .013 to .014 but the sound is more natural, sweeter, rounder and, interestingly, more spacious. It is really noticeable in high register or sibilants. I can no longer hear "shh", just a sharp "s". I guess I can pay the thermal price for those benefits..
I'm also about to start over again with the decoupling mod which I also reversed. The sound was almost to sharp and clinical which made me tiered after an hour of listening. Now I have proper quality parts to do it so let's see how it will turn out.
More to the point ethernet RJ45 jacks wouldn't work with litz wire since they use insulation displacement and require uninsulated strand(s) within the PE insulation to make contact. Litz strands have to be insulated but also have to be commoned reliably at each end.
Litz wire is mainly used in inductors and transformers where the thermal advantage of reduced losses (and overall efficiency increase) can make it worthwhile and the operating frequency is in the 10kHz to 1MHz range.
Its not so good above 2MHz due to parasitic capacitance between the strands, apparently, so inappropriate for most data comms uses such as ethernet.
It turned out quite well
I put 100R Dale CMFs and 100uF Nichicon KZ on the +B3 and 33uF Nichicon KZ on the -B1. While Nichicons are grounded as usual, I left 0.1 uF poli bypass caps as they appear on the schematic. Last time I got rid of them altogether and maybe that's why the amp sounded too cold and clinical.This mod definitely changes the tone signature of the amp. While stock, it sounds a little bit veiled, more round around the edges, compressed and massive, with the emphasis of low mids and bass in the overall tone. After decoupling mod, the amp became pleasingly sharp and clear, with much more open and precise highs. Also the tone appears more midrange and top end focused with low end less compressed, less striking. I guess it's just more neutral now.
In my case, I can't get the THD lower than 0.014% and THD+N lower than 0.022%, @ 10W into 8R load.
I think I'm done for a while with tinkering. Let's listen to some music!
Overhaul & upgrade Rotel RB-991
Hi there,
After reading this thread a few months ago i reached out to Per and asked him if he could do something for my 2 Rotel’s RB-991's.
After 20 years of amplifying music they needed some tlc.
Per gave the tech manual a look and told me that this amp had a completely different design then the amps he was working on. He was very busy so he asked me to go ahead with the planned upgrades and be patient for a while.
I went ahead with replacing the psu caps, rectifiers and the electrolytics on both channels.
The aftermarket psu caps where bought from DNM which stands for Dennis Neil Morecraft, he has the patent for the slitfoil capacitor. The caps are made by Supertech to DNM's specifications. Stock was BHC 80Vdc 15000uF, the DNM's do 10000uF so i replaced 1 BHC for 2 from DNM.
Stock rectifier(s) pulled out and replaced with 3 x STPS60SM200CW Schottky diodes. Psu wiring to the channels was replaced to 6 mm2 copper litze with crimped copper terminals.
Toroidal trafo and psu caps bank where placed on separate 4mm thick aluminium sheet panels with vibration dampers connected to the chassis to minimize vibration influences.
Electrolitics:
C601/602 50V 4,7uF Black Gate replaced by : Mundorf Mcap EVO 1uF
C607/608 25V 100uF Black Gate replaced by : 2x Nichicon BP 50V 47uF
C611/612 50V 10uF Black Gate replaced by : Nichicon FG or Elna SilmicII 50/10
C623/626 100V 1000uF Nichicon VX(M) replaced by : Rubycon YXG 100/1000
I started with Wima’s on 601/602 and 611/612 but ditched them because i didn’t like the sound.
I haven’t made up my mind yet soundwise on the Nichicon’s or Elna’s on 611/612.
After a few weeks i reached out to Per that i did my job and the amps where playing and settled for about 60 hours.
Hi there,
After reading this thread a few months ago i reached out to Per and asked him if he could do something for my 2 Rotel’s RB-991's.
After 20 years of amplifying music they needed some tlc.
Per gave the tech manual a look and told me that this amp had a completely different design then the amps he was working on. He was very busy so he asked me to go ahead with the planned upgrades and be patient for a while.
I went ahead with replacing the psu caps, rectifiers and the electrolytics on both channels.
The aftermarket psu caps where bought from DNM which stands for Dennis Neil Morecraft, he has the patent for the slitfoil capacitor. The caps are made by Supertech to DNM's specifications. Stock was BHC 80Vdc 15000uF, the DNM's do 10000uF so i replaced 1 BHC for 2 from DNM.
Stock rectifier(s) pulled out and replaced with 3 x STPS60SM200CW Schottky diodes. Psu wiring to the channels was replaced to 6 mm2 copper litze with crimped copper terminals.
Toroidal trafo and psu caps bank where placed on separate 4mm thick aluminium sheet panels with vibration dampers connected to the chassis to minimize vibration influences.
Electrolitics:
C601/602 50V 4,7uF Black Gate replaced by : Mundorf Mcap EVO 1uF
C607/608 25V 100uF Black Gate replaced by : 2x Nichicon BP 50V 47uF
C611/612 50V 10uF Black Gate replaced by : Nichicon FG or Elna SilmicII 50/10
C623/626 100V 1000uF Nichicon VX(M) replaced by : Rubycon YXG 100/1000
I started with Wima’s on 601/602 and 611/612 but ditched them because i didn’t like the sound.
I haven’t made up my mind yet soundwise on the Nichicon’s or Elna’s on 611/612.
After a few weeks i reached out to Per that i did my job and the amps where playing and settled for about 60 hours.
Attachments
Overhaul & upgrade Rotel RB-991
Part 2:
Then it all started to get very technical for me as Per guided me through the amps and asked me to take measurement (Vdc across) on resistors R629/623/630/624 (100R) at VAS transistors Q615/613/616/614.
The VAS transistors where very hot to the touch and that showed in the measurements, almost twice the voltage as was calculated!
Heatsinks where dissipating 1,1 Watt but are designed to handle 0,5!!
Per recalculated the value for the resistors to 196R but we settled on 187R 1% 0.5W metal film.
I also ordered new multiturn 2.2K 0.5W trimmers for the bias, the old single turn trimmers where very difficult to set.
After i did this job the VAS was indeed running cooler, warm, not lukewarm but warm.
This made the amps already sound better, like without effort, less restraint….
Per warned me about the next step because it would affect the much loved Rotel sound.
By design the VAS is heavily loaded to create this Rotel sound.
It is done with C609/610, a 330pF with a 33K resistor parallel.
Both of them where replaced with 1 x 15pF 100V ceramic npo cap.
Miller caps where placed between VAS base and collector, 2 x 47pF 100V ceramic npo cap.
After i did this upgrade i was warned about oscillation and had to measure Vac between output terminals before listening to music.
There was no Vac measured so i switched them both on and was in for a big audiophile surprise.
The Rotels sound different but for me in a very positive way.
They are very clear now, the “fat” Rotel sound is gone and replaced for something more clinical.
The highs are very crisp, clear as running water.
The mids i need getting used to because they are a big factor in the Rotel sound.
The lows are still there, not “fat” anymore but more subtle but very much in control.
The clarity delivers more definition in the soundstage as loads of details and dynamics in music.
Music i know very well surprises me now because i hear so much more details of instruments i never heard before, wow.
It is right in your face now, focussed, voices are deadcenter and the soundstage is broader and deeper then before, real 3D!
I'm quite proud of what i did because i'm not educated in electronics at all, i know how to solder though and not afraid to rip a big amp apart into its components so i would like to offer a very big thank you to Per for his knowledge and patience with me
My stereo set is composed of a NAS to a Rpi3b (Volumio software) with ALLO Isolator and Kali attached, I2s to Doede Douma 1794 NOS dac, output to Silk tvc’s connected to 2 Rotel RB-991’s (Bi-amping) connected to Chario Millenium II
Part 2:
Then it all started to get very technical for me as Per guided me through the amps and asked me to take measurement (Vdc across) on resistors R629/623/630/624 (100R) at VAS transistors Q615/613/616/614.
The VAS transistors where very hot to the touch and that showed in the measurements, almost twice the voltage as was calculated!
Heatsinks where dissipating 1,1 Watt but are designed to handle 0,5!!
Per recalculated the value for the resistors to 196R but we settled on 187R 1% 0.5W metal film.
I also ordered new multiturn 2.2K 0.5W trimmers for the bias, the old single turn trimmers where very difficult to set.
After i did this job the VAS was indeed running cooler, warm, not lukewarm but warm.
This made the amps already sound better, like without effort, less restraint….
Per warned me about the next step because it would affect the much loved Rotel sound.
By design the VAS is heavily loaded to create this Rotel sound.
It is done with C609/610, a 330pF with a 33K resistor parallel.
Both of them where replaced with 1 x 15pF 100V ceramic npo cap.
Miller caps where placed between VAS base and collector, 2 x 47pF 100V ceramic npo cap.
After i did this upgrade i was warned about oscillation and had to measure Vac between output terminals before listening to music.
There was no Vac measured so i switched them both on and was in for a big audiophile surprise.
The Rotels sound different but for me in a very positive way.
They are very clear now, the “fat” Rotel sound is gone and replaced for something more clinical.
The highs are very crisp, clear as running water.
The mids i need getting used to because they are a big factor in the Rotel sound.
The lows are still there, not “fat” anymore but more subtle but very much in control.
The clarity delivers more definition in the soundstage as loads of details and dynamics in music.
Music i know very well surprises me now because i hear so much more details of instruments i never heard before, wow.
It is right in your face now, focussed, voices are deadcenter and the soundstage is broader and deeper then before, real 3D!
I'm quite proud of what i did because i'm not educated in electronics at all, i know how to solder though and not afraid to rip a big amp apart into its components so i would like to offer a very big thank you to Per for his knowledge and patience with me
My stereo set is composed of a NAS to a Rpi3b (Volumio software) with ALLO Isolator and Kali attached, I2s to Doede Douma 1794 NOS dac, output to Silk tvc’s connected to 2 Rotel RB-991’s (Bi-amping) connected to Chario Millenium II
Attachments
Well, I'd just like to say well done to Smartdriver for his amazing job in taming these big RB-991 beasts.
His stereo setup consists of some devices I have never even heard of before, but they definitely sounds to be on the expensive side.
On that background it is amazing that changing a few resistors and capacitors costing a few pence can actually make a big audible difference.
But mainly, it is the words "wow" and "so proud that I actually did this" that is at the very heart of this diyAudio community and what makes all this modding/upgrading stuff so satisfying.
And to quote PakaPL above: "done for a while with tinkering. Let's listen to some music!"
How true.
Per
His stereo setup consists of some devices I have never even heard of before, but they definitely sounds to be on the expensive side.
On that background it is amazing that changing a few resistors and capacitors costing a few pence can actually make a big audible difference.
But mainly, it is the words "wow" and "so proud that I actually did this" that is at the very heart of this diyAudio community and what makes all this modding/upgrading stuff so satisfying.
And to quote PakaPL above: "done for a while with tinkering. Let's listen to some music!"
How true.
Per
Rotel RB-980BX/981 mods
I’m using the subject amps in a tri-amp config. The 981 drives the mids & tweeters; the 980 drives a pair of woofers. If you have tips on parts selection to upgrade:
RB-980BX: focus is bass - LP’d ~120hz though 200hz is possible
RB-981: focus is mids, bandpassed 120hz & ~2Khz; tweeter HP’d ~2Khz
...I would be very grateful. I know the 990 series amps have been compared to Krell and Levinson w/out the mods. OTOH I’ve read the 980/981 called “baby 990s.”
I confess speed is not my forte’. I’ll start w/ studying your work very carefully. That and obtain schematics - yup when I do I’ll come back up on the net.
After I finish these, two each, a RB-985 is next. 5 100 Watts - considered by some a 5 ch 980BX.
I admire DIYers. I’ve done a little. Thanks in advance for your help & reading this far. Tony
Did your damping improve? You’re man after my heart w/the PS & internal wiring upgrades and everything inspired me to get busy.Part 2:
Then it all started to get very technical for me as Per guided me through the amps and asked me to take measurement (Vdc across) on resistors R629/623/630/624 (100R) at VAS transistors Q615/613/616/614.
The VAS transistors where very hot to the touch and that showed in the measurements, almost twice the voltage as was calculated!
Heatsinks where dissipating 1,1 Watt but are designed to handle 0,5!!
Per recalculated the value for the resistors to 196R but we settled on 187R 1% 0.5W metal film.
I also ordered new multiturn 2.2K 0.5W trimmers for the bias, the old single turn trimmers where very difficult to set.
After i did this job the VAS was indeed running cooler, warm, not lukewarm but warm.
This made the amps already sound better, like without effort, less restraint….
Per warned me about the next step because it would affect the much loved Rotel sound.
By design the VAS is heavily loaded to create this Rotel sound.
It is done with C609/610, a 330pF with a 33K resistor parallel.
Both of them where replaced with 1 x 15pF 100V ceramic npo cap.
Miller caps where placed between VAS base and collector, 2 x 47pF 100V ceramic npo cap.
After i did this upgrade i was warned about oscillation and had to measure Vac between output terminals before listening to music.
There was no Vac measured so i switched them both on and was in for a big audiophile surprise.
The Rotels sound different but for me in a very positive way.
They are very clear now, the “fat” Rotel sound is gone and replaced for something more clinical.
The highs are very crisp, clear as running water.
The mids i need getting used to because they are a big factor in the Rotel sound.
The lows are still there, not “fat” anymore but more subtle but very much in control.
The clarity delivers more definition in the soundstage as loads of details and dynamics in music.
Music i know very well surprises me now because i hear so much more details of instruments i never heard before, wow.
It is right in your face now, focussed, voices are deadcenter and the soundstage is broader and deeper then before, real 3D!
I'm quite proud of what i did because i'm not educated in electronics at all, i know how to solder though and not afraid to rip a big amp apart into its components so i would like to offer a very big thank you to Per for his knowledge and patience with me
My stereo set is composed of a NAS to a Rpi3b (Volumio software) with ALLO Isolator and Kali attached, I2s to Doede Douma 1794 NOS dac, output to Silk tvc’s connected to 2 Rotel RB-991’s (Bi-amping) connected to Chario Millenium II
I’m using the subject amps in a tri-amp config. The 981 drives the mids & tweeters; the 980 drives a pair of woofers. If you have tips on parts selection to upgrade:
RB-980BX: focus is bass - LP’d ~120hz though 200hz is possible
RB-981: focus is mids, bandpassed 120hz & ~2Khz; tweeter HP’d ~2Khz
...I would be very grateful. I know the 990 series amps have been compared to Krell and Levinson w/out the mods. OTOH I’ve read the 980/981 called “baby 990s.”
I confess speed is not my forte’. I’ll start w/ studying your work very carefully. That and obtain schematics - yup when I do I’ll come back up on the net.
After I finish these, two each, a RB-985 is next. 5 100 Watts - considered by some a 5 ch 980BX.
I admire DIYers. I’ve done a little. Thanks in advance for your help & reading this far. Tony
Last edited:
RB-980BX upgrades
Hi Tony,
I have actually recently repaired and upgraded a RB-980BX that had similar issues as Smartdriver's 991.
The 980 is actually a bit easier to modify as the pcb is more accessible and I could place emitter followers on the VAS stage.
I enclose the schematic that I finally decided on and my report on what was done and achieved FYI.
Cheers,
Per
Hi Tony,
I have actually recently repaired and upgraded a RB-980BX that had similar issues as Smartdriver's 991.
The 980 is actually a bit easier to modify as the pcb is more accessible and I could place emitter followers on the VAS stage.
I enclose the schematic that I finally decided on and my report on what was done and achieved FYI.
Cheers,
Per
Attachments
Ok. So adding:
2 transistors - BTW what does the emitter follower do
2 resistors
5(?) caps
Any other upgrades or additions for the font end?
Power supply upgrades:
4 Smaller caps/channel
Bridge rectifiers adequate? Or high speed diodes?
Any PS additions?
BTW I did download & print the 980 & 981 schematic. Will attempt an upload at earliest opportunity
I have questions on Smartdriver’s upgrades but I’m not home right now.
1000 thank 😊. Tony
2 transistors - BTW what does the emitter follower do
2 resistors
5(?) caps
Any other upgrades or additions for the font end?
Power supply upgrades:
4 Smaller caps/channel
Bridge rectifiers adequate? Or high speed diodes?
Any PS additions?
BTW I did download & print the 980 & 981 schematic. Will attempt an upload at earliest opportunity
I have questions on Smartdriver’s upgrades but I’m not home right now.
1000 thank 😊. Tony
Hi Tony,
The emitter followers function like a buffer or (sort of) like a unity gain (x1) opamp. They provide the same ac signal from input to output, but have a high input impedance and a low output impedance. This reduces the VAS transistors' loading of the LTP input pairs and significantly reduces distortion.
I have designed two types of small modules, a current mirror (CM1) and a VAS3 which do it all (includes all the transistors, capacitors and resistors) and are much easier to install, but the very high rail voltages in the 980/991 would make them run too hot.
I am working on an idea (which Smartdriver is also interested in), and will be testing it on a RA-971 which I have had on my to-do list for too long.
So, if you have a bit of patience there may a workaround available soon.
Re: Power Supply, I believe the key is to have a high capacity and a low ESR (internal resistance) as possible so the reservoir caps can deliver the power required quickly.
Multiple caps do do that, but you need the space for them.
I wouldn't put in high speed diodes, after all they are only rectifying 60 Hz (50 Hz in our money). Even that requires a set of snubbing caps to slow down and reduce the diode switching noise.
But I know that some disagree with me on that point.
Cheers,
Per
The emitter followers function like a buffer or (sort of) like a unity gain (x1) opamp. They provide the same ac signal from input to output, but have a high input impedance and a low output impedance. This reduces the VAS transistors' loading of the LTP input pairs and significantly reduces distortion.
I have designed two types of small modules, a current mirror (CM1) and a VAS3 which do it all (includes all the transistors, capacitors and resistors) and are much easier to install, but the very high rail voltages in the 980/991 would make them run too hot.
I am working on an idea (which Smartdriver is also interested in), and will be testing it on a RA-971 which I have had on my to-do list for too long.
So, if you have a bit of patience there may a workaround available soon.
Re: Power Supply, I believe the key is to have a high capacity and a low ESR (internal resistance) as possible so the reservoir caps can deliver the power required quickly.
Multiple caps do do that, but you need the space for them.
I wouldn't put in high speed diodes, after all they are only rectifying 60 Hz (50 Hz in our money). Even that requires a set of snubbing caps to slow down and reduce the diode switching noise.
But I know that some disagree with me on that point.
Cheers,
Per
Hi there Tony,
Thanks for the kudos but i only did the physical work so to speak, the real technical stuff came from Per, i owe him a beautiful sounding pair of Rotel amps.
As i said before, i'm not afraid to tear equipment down and rebuild it but replacing parts for different values let alone design circuitry is very much out of my league
, i know how to use a soldering iron, i recognize various components and i can take measurements, thats it
But if you have questions, i'll be happy to answer them to the best of my ability.
this is a unticked box on the "to do list".
I read somewhere that you don't need to mount them on every diode because the ringing happens in the secondary of the transformer, so use one snubber on the secondary. Does this make sense to you?
Cheers, Paulus
Thanks for the kudos but i only did the physical work so to speak, the real technical stuff came from Per, i owe him a beautiful sounding pair of Rotel amps.
As i said before, i'm not afraid to tear equipment down and rebuild it but replacing parts for different values let alone design circuitry is very much out of my league
, i know how to use a soldering iron, i recognize various components and i can take measurements, thats it But if you have questions, i'll be happy to answer them to the best of my ability.
I agree Per
this is a unticked box on the "to do list".I read somewhere that you don't need to mount them on every diode because the ringing happens in the secondary of the transformer, so use one snubber on the secondary. Does this make sense to you?
Cheers, Paulus
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