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Old 17th August 2008, 06:37 AM   #21
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Default Re: Update.

Quote:
Originally posted by mikesnowdon
I had a snooze this afternoon which lasted 4 hours so I've been up late working on the amp.

Heres a list of the changes:

Connected the IEC earth terminal to chassis, and shortened the wires from IEC to Transformer.

Made a mini star ground at the input ground, this connects to the negative speaker terminals and connects to the PCB ground @ the input solder pad, all wired with solid core copper I stripped from some T.V. coax.

Removed the negative return wires from the binding posts to the PCB. Only positive wires run from PCB to positive terminal using 1.5mm stranded copper milspec wire.

Re-wired the positive input connections with a twisted pair each of .5mm milspec stranded copper wire (got that from work fro nowt!).

Changed the input caps to 10uF Oscons and bypassed under PCB with .22 wima foils.

Changed feedback caps to 100uF Tants.

Theres caps that link from pins 14, into the output, (not sure why?) I changed them to 100uF Rubycon ZLH.

I taped the shaft of the volume pot in a crude attempt to isolate it from the knob.

The above has made a difference to the sound in a positive way.

The midrange and treble has more detail, the overall sound is cleaner, and theres no hum when I touch the vol knob! The only thing is now there seems to be slightly less bass, its still there and its not bad, just a little quieter than before. Maybe adding another 4700uF cap will help as well as upping the value slightly (and quality) of the output caps. Its late night so I cant turn the wick up. I hope after a couple of days the bass will come back and the sound will get warmer as things settle in. Initial listening is positive and im quite happy with the improvement.

However, there is a strange thing thats happened which I can see the cause of. The speakers were out of phase. This was easy to solve by swapping the plugs on one speaker. Maybe I wired my speakers wrong when I changed the internal wire. who knows, its wierd. I'll check them later. Nevertheless, Im happy with what I've done and will let you all know how things sound once settled in.

I'd still like some advice on a reasonably good pot to use please.

Thanks all,
Mike.
Its usually advisable to observe the effects of just one change at a time. You may need to review them, one at a time.
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Old 18th August 2008, 12:38 AM   #22
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Indeed, Im sure I should have done that in hindsight. However, I felt the cheap lytics had to go so went with my gut feeling on the feedback caps. I'll give things a day or 2 more to settle in and see how it sounds.

I'm using an old Marantz CD-46 as source connected via the headphone jack with a bodged interconnect made from a QED Quinnex 2. As I didnt have a 3.5mm stereo jack I used one off some old headphones.

Tonight I decided to re build it with a nutrick 3.5 jack. As the PC (and Marantz) arent earthed I decided to connect the shield at both ends. Soldered everything with Eichmann silver solder and used teflon tubing to insulate the sheild leadouts. It seems to have made a slight difference. Right now im listening to Pink Floyd 'Delicate sound of thunder' and the sound is rather good. I still have the issue of weak bass though. I suspect its due to the cap cahnge.

The input cap (and bypass) has a combined capacitance of 10.22uF. This should be plenty as the datasheet for the TEA2025 recomends 0.22uF minimum. I have 10uf OSCONS here with .22 Wimma's as bypass.

The feedback cap, though a different type (Tant bead) is of the same value as the original, so I cant see how this would effect the bass too much.

The same goes for the caps that link into the output. Same value just changed fron cheap lytics to Rubycon ZLH.

I'll try a few different combinations and also give the 100uF Nichion muse a try in different areas. Other than that, all thats left to try is adding capacitance to the main PSU resovouir cap. I dont think its worth adding high end resistors of upgrading the diodes. It probably wouldnt make a noticeable difference with this chip.

Stil looking for advice on a reasonable pot to use?
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Old 18th August 2008, 06:24 AM   #23
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Quote:
Originally posted by mikesnowdon
Indeed, Im sure I should have done that in hindsight. However, I felt the cheap lytics had to go so went with my gut feeling on the feedback caps. I'll give things a day or 2 more to settle in and see how it sounds.

I'm using an old Marantz CD-46 as source connected via the headphone jack with a bodged interconnect made from a QED Quinnex 2. As I didnt have a 3.5mm stereo jack I used one off some old headphones.

Tonight I decided to re build it with a nutrick 3.5 jack. As the PC (and Marantz) arent earthed I decided to connect the shield at both ends. Soldered everything with Eichmann silver solder and used teflon tubing to insulate the sheild leadouts. It seems to have made a slight difference. Right now im listening to Pink Floyd 'Delicate sound of thunder' and the sound is rather good. I still have the issue of weak bass though. I suspect its due to the cap cahnge.

The input cap (and bypass) has a combined capacitance of 10.22uF. This should be plenty as the datasheet for the TEA2025 recomends 0.22uF minimum. I have 10uf OSCONS here with .22 Wimma's as bypass.

The feedback cap, though a different type (Tant bead) is of the same value as the original, so I cant see how this would effect the bass too much.

The same goes for the caps that link into the output. Same value just changed fron cheap lytics to Rubycon ZLH.

I'll try a few different combinations and also give the 100uF Nichion muse a try in different areas. Other than that, all thats left to try is adding capacitance to the main PSU resovouir cap. I dont think its worth adding high end resistors of upgrading the diodes. It probably wouldnt make a noticeable difference with this chip.

Stil looking for advice on a reasonable pot to use?
The feedback cap has an large effect on audio--exactly as much as the input filter (dc block) cap.

When using a bypass cap to a larger cap that's carrying an audio signal, its good to use a "necessary" bypass cap otherwise the results are frequenty either a decrease of clarity or a loud midrange boost, or both. With plastic or ceramic caps, one can use a smaller than anticipated size.

For either case, and more, its possible that you may have arranged for a midrange boost rather than decreased bass.

A reasonable pot is alpha A20k Dual Gang (audio type stereo) and its very cheap. Between that and the more expensive pots are differences of manufacturing variance and longevity.
However. . .
Over at Decibel Dungeon http://myweb.tiscali.co.uk/nuukspot/...e.html#gcprevc is a nice spot to tell you how to do a "law fake" for perhaps a B50k Dual Gang pot (linear), turning it into a 10k to 20k (or so) audio type. A law fake is merely an additional load on the "amplifier side" of a linear pot.
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Old 25th August 2008, 01:03 AM   #24
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Default Pots.

I found 10k and 20k Alps pots on ebay going cheap, is it worth it?

Another option would be to use the pot I removed from the tone circuit as its identical.

Soundwise the amps producing quite nice sound for what it is. It seems to lack punch in the bass though, may be due to the cap changes. Im going to be paitent and see if things change after 1 more week. I decided to get the PSU kit from audioupgrades.co.uk. For the money its great value: PCB, Schottky diodes , 4700uF Panasonic FC, 220uF Rubycon ZA, and a 78/9** reg. All for under 20. I'll only use the diodes and 4700uF cap to begin with, adding the one I allready have to double up the smoothers. If I order the kit with 7809 reg I can try adding that later for experimentation.

I think first I should better understand the chip.

I need to be sure that the feedback caps are the middle 2, and I need to know what the last pair are doing (the ones that link into the output)??????
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Old 25th August 2008, 05:42 AM   #25
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Caps:
Capacitors don't change their sonic signature greatly after break-in. There are few exceptions. For most caps, if it doesn't sound better in 15 minutes, its not going to, and if it sounds bad immediately, it won't stop doing that; however, if it sounds good immediately, then it will probably keep doing that.

So, if you've made a no-bass arrangement, then the task is to figure out which one of the changes or which several of the changes was responsible for that modification. This won't suddenly change (back) by itself. The power supply, feedback cap, output cap, and input filter caps are suspect as the cause.

Hotrod with caps is possible:
For increasing the impact of an audio AC pulse, there are some capacitors made for it. Those that help break the drywall in nightclubs everywhere are also good for hot-rodding fun. That's Nichicon's ES, Blackgate's N, Elna's Cerefine, and very few others. Although made for audio signal, most also do amazing things in power supplies.
All of these are more difficult to apply than a plain wire, because, with any capacitor, your application may not work with the ideal bandwidth of your purchase.
If an input filter cap misses the high end slightly, there's the "bypass cap" practice for hours of tinkery fun. Its also possible to put a resistor (avg value 20R) onto the input side of a very small value MKT bypass cap (avg value 180nF) in order to sink the mids without reducing perceived clarity--eventually resulting in more apparent bass response.

Volume control:
Alps makes some of the best potentiometers and some of the worst too. The value of a volume control is the technology inside, successfully applied to the application.
I'm pretty sure that Alpha's cheap carbon beats Alps cheap carbon by a landslide, so I wouldn't pay any more than $2 for a carbon pot.
However, Alps also makes pots from more advanced materials like plastics, ceramics and perhaps pixie dust? Many other manufacturers also have advanced materials options.
A step ahead of the carbon or plastic pots is the "stepper" style, involving a rotary switch and a lot of good quality resistors.
Next up are the more interesting technologies of the lightbulb>CDS, the Joshua, and the transformer based volume controls that don't cost more than the highest "zoot factor" pot, but can outperform them all (depends on application).
The usage of the highest quality volume controls is somewhat regulated by system symmetry (whether or not it benefits the application) although quality input is always a good starting place.

Testing:
In theory, speakers are an awful thing to test with for home projects; however, headphones can do well for testing amplifiers. If using headphones to test, then, next time you move or buy new speakers, then you don't get the task of re-doing the amplifier. Here's a great article by Rod Elliot: http://sound.westhost.com/project100.htm
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Old 29th October 2008, 11:03 PM   #26
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Just thought I'd end off on this one and post a final piccy of the amp as it is now.

I rescued the Pana FC'c and fitted a 4700uF Rubycon PK. Added some Oscons for signal decoupling duties. Left the rest of it including the ALPS pot as it was secondhand anyway. Theres still quite a few nice caps in there which I'll probably salvage when I need them for more worthy equipment. The amp sounds allright though, not hi-fi but listenable and it suits the Missions for nearfield listening. It has a groovy nature and being used nearfield it relly grabs your attention. Surprisingly it does draw you in to the music.......(as long as you dont push it too much - distortion)

As I said, this project was fun and I learnt from it. I had a ball with it.

I thought of a name for it: OTTA (as in otter, small and cute, OTTA also stands for something - Over The Top Amp! )


Click the image to open in full size.

Click the image to open in full size.

And this is what I started with:

Click the image to open in full size.
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Old 29th October 2008, 11:21 PM   #27
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Forgot to mention.

I had experimented with a adding a second 4700uF Panasonic FC. I was hoping for better bass but what I got was overly revealing midrange. With a better quality chip the additional cap might have had a positive effect. I opted for the Rubycon in the end as it works better with the chip. I also added shottkey diodes which improved the dynamics and made the bass better defined. The ground now runs from the star point at the terminals to the negative side of the main smoother. This cleaned up the sound a little.

I still have a hankering to try putting the Tants back and possibly add a small 100uF Bypass cap on the main smoother at some point. But for now Im enjoying a very listenable sound. Its not the smoothest but it has good timing and reasonable detail. Its a little bright though so I might pad down the tweeters in the speakers in using. One thing I really like is the punchy nature it has. Basslines really get you moving.

I think the grainy sound is due to the chip and all the good components are revealing its shortfalls. My inkling is that putting the tants back in for the feedback and output (currently using Rubycon ZLH) will warm things up a bit.
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Old 29th October 2008, 11:22 PM   #28
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Kudos!!! Its a very cute amplifier!
I'm thinking of making one somewhat like it.
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Old 29th October 2008, 11:23 PM   #29
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Thanks man!
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Old 13th December 2008, 10:57 PM   #30
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I tried putting the Tants back in the feedback, added warmth but also gave me a distorted top end. Replaced the Rubycon ZLH's in the feedback and tried the Tants on the bootstrap. Also I added a 220uF Rubycon ZL to the main smoother.

Much better sound was achieved! Detail seperation has improved nicely. Top end extension is better and more detailed. The amp seems to do a better job at reproducing the 'air' of instruments. Bass is more natural, still goes quite deep but has better punch now, tighter. The amp sounds a bit toppy though but this could be because I have my Mission speakers upside down. I'll have a bit more of a play with the speakers later. (Planning to mod them a bit sometime. Polyprop caps and new resistors (probably a slightly higher value on the tweeter).

Might try 100uF Tants on the input with the Wimma polyprop bypass caps. Currently I have 10uF Oscons with Wimma bypass. Adding the Larger value Tants might improve bass and give a bit of extra warmth to the sound.....

Overall there seems to much better clarity and air to the sound, and detail has improved further. Im happy.

Click the image to open in full size.

Mike.
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