DSE A2760 Amplifier (aka Koda-261) - Page 2 - diyAudio
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Old 12th April 2010, 02:40 AM   #11
ratfink is offline ratfink  Australia
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Thanks for posting details of the modifications, Reszipsa. I recently managed to get my hands on two of these amplifiers (one on hard rubbish and another on e-bay) and was planning to use them as part of a bi-amp active set-up. Just wondering whether you think it's worth modifying these, or am I better off using the cases and p/s etc as the basis of new amps?
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Old 30th July 2010, 12:48 PM   #12
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Provided the second hand amps you've bought are in (physically) good condition, you should mod them. The cost is minimal if you do the work yourself, and the improvement in performance is quite dramatic.

Alternatively, you could ask Elson Silva to mod the amps for you. He can be contacted through the NSW Audiophile Society.

As the output stage of this amp uses only a single pair of bipolar transistors, I would recommend that it not be used with low sensitivity/low impedance speakers.
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Old 25th May 2012, 09:29 AM   #13
P10K is offline P10K  Australia
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Sorry to bump an old thread.

I've got one of these amps and am aware that changing some caps etc on the board and drastically improve the amp.

Whilst I can solder, i know little about electronics and have no idea which caps etc i should be upgrade and with what.

Is there a guide or could someone explain to me what needs to be changed/why?

Phil
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Old 25th May 2012, 10:06 PM   #14
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The "what" is in Post #8.

For reference, here's the schematic of the original unit.

Looking at the mods suggested, I'm not sure whether I can agree with all of them.
I'm not sure what the deal with the .22R wirewounds is. Connected as described, you'd end up with a lousy damping factor of only 9 re: 4R (18 re: 8R), which is considerably less than I'd consider acceptable in a "serious" amplifier and would be expected to lead to audible coloration with many a real speaker.
It also seems the intent is to use the unit as a power amp only. If you don't want that, I'd just tweak the preamp feedback network a little for more input headroom (R132/135: 4k7 MF, R139/143: 10k MF, C134/139: 68p ceramic NP0) and be done with it. In the power amp, R230/253 can then retain their original values, save for changing to metal film, and R231/281 should become 47k for lowest DC offset (assuming you do not install the 39k parallel resistor at R238/286 they suggest). You can use a half-watt part for R231/281 if you want to get fancy.
A few more items are mysterious or questionable at best. Why replace R239/287 by noisier 10k units (I'd rather suggest 1k, and 1/4 W will do fine)? Why relocate R238/286 (essentially no functional difference at all)?

The basic ailment of the power amp seems to be that the output transistors are not in the feedback loop (WTF?) and accordingly, there is serious distortion. (Looking at the stock distortion graph, performance is just garbage.) If you do want to include them, extra compensation has to be added for stability (a fancier 2-pole one in this case). They're also adding trimpots for cranking up quiescent current (for even lower distortion at low power) and reducing voltage gain from 36 dB to 26 dB (for use as a "standard" power amp) here.

I guess I'll have to play with the simulation model posted earlier to determine stability and all that good stuff.

Last edited by sgrossklass; 25th May 2012 at 10:17 PM.
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Old 3rd July 2012, 07:49 AM   #15
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The 022R resistors are intended to reduce the damping factor to produce a somewhat "warmer" sound quality. A number of people who have auditioned the modded amp have expressed a preference for the sound in this configuration that with the stock output inductors.

An alternative is to retain the stock inductors and shunt each with a 1R/10W resistor. This prevents degradation of the damping factor and minimises "ringing" with capacitive loads.

All of the suggested mods for the amp were derived from extensive SPICE modelling, even though some may seem inexplicable or even sub-optimal at first blush.

BTW, while the amp is stable with a pair of series-connected 0R22 resistors between the output transistors and the speaker load, the margin of stability may not be very great under certain load conditions.

I therefore recommend that either a 1R/10W resistor be substituted for each series pair of 0R22 resistors (resulting in an even lower damping factor, which may not be subjectively acceptable to some) or that the stock output inductors be left in place and shunted with 1R/10W resistors.

IMO, the preamp section of the amp degrades the sound quality, and should just be bypassed. I suspect that tweaking the preamp's feedback network will be like putting a Band-Aid on a gangrenous leg
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Old 25th April 2013, 11:21 AM   #16
TMM is offline TMM  Australia
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FWIW, here is the measured performance of the power amp (completely standard - no preamp) in one of these amps @ 1W into 8ohm:
1Khz:
Click the image to open in full size.
667Hz:
Click the image to open in full size.

My measurement setup is nothing special - USB DAC (TE7022+ES9023) as source, Asus Xonar DX as ADC. If the distortion from the amp disappears below the noise floor after modding i'll be pretty happy
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Old 2nd May 2013, 10:19 AM   #17
TMM is offline TMM  Australia
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Complete list of mods I've decided on:
-Replace C206/C224, C207/C231 with Panasonic FC 470uF 63V
-Replace C204/C228 with Panasonic FM 100uF 50V
-Replace C208/C234 with NP 100uF 50V (two series Panasonic FC 220uF 63V connected neg-neg)
-Replace C212/C240, C205/C233 with MKP 4.7uF (Low ESR Polarised Electro is fine for C205/C233, i just have MKP caps on hand already)
-Replace R230/R253 with 3K||2 metal film 1% 0.5W
-Double pole filter, 220pF C0G/100pF C0G/2.2K metal film 1% 0.5W. 100pF to base of V215/V235, 220pF to collector of V214/V232, 2.2K to rail side of R232/R278.
-Replace R231/R281 with 120K||4 metal film 1% 0.5W
-Disconnect R231 from junction of R224/R225. Connect R231 to input side of L201
-Disconnect R281 from junction of R270/R272. Connect R281 to input side of L202
-Shunt R209/R257 with 1K multiturn trimmers, adjust quiescent current as desired.

Removing the smaller caps probably isn't a great idea imo. They are there for stability and shouldn't have any effect on SQ.
I'm also not sure that a resistor across L201/L202 (~1.2uH) is absolutely necessary nor offers any SQ advantage - resonance is way out of band and should only occur if the amp is oscillating (which it shouldn't!).
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Old 9th May 2013, 02:40 PM   #18
TMM is offline TMM  Australia
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After adding the 1K trimmers the amp was already idling at ~200mA quiescent current so i just left it as is.

I remeasured (Xonar in loopback this time), did the mods above and measured again
Standard:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

Mod:
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

So distortion performance has gone from ~0.04% to ~0.0023% @ 1watt. gotta be happy with that One odd thing i noticed is that R222/R268 were connected to the INPUT side of L201/L202 contrary to the schematic. Not sure if that was intentional or a mistake on the PCB. Doesn't seem to affect performance.

Last edited by TMM; 9th May 2013 at 02:55 PM.
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Old 9th May 2013, 11:37 PM   #19
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hi TMM,

Thanks for posting your interesting results. Looks like the mods where worthwhile.

regards
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Old 5th June 2013, 12:35 PM   #20
TMM is offline TMM  Australia
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No worries Greg

I managed to get my hands on two more of these amps - they have a stylised font on the front panel silkscreening and the DSE logo is silkscreened instead of a badge.
The internal construction is the same - only a few capacitors differ in brand.

I thought it would be interesting to see what performance was like with the preamp still installed. Here is 1W @ 8ohm:
Click the image to open in full size.
(ignore the THD measurement, it's wrong :P, correct result is 0.29% THD)
Looks like the transformer might be better on this amp, either that or my mains supply was cleaner during this test (haven't really looked into it). Distortion is significantly worse than my first amp, i measured R212/R262 and found 19mV on one channel and 10mV on the other. This equates to 68mA and 36mA bias currents - seems low. The above measurement happens to be from the channel biased at 36mA. Both of the 'new' amps also run very cool at idle accordingly - my first amp ran warm even before mods.

Last edited by TMM; 5th June 2013 at 12:43 PM.
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