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Old 28th December 2012, 03:55 PM   #41
Jay is offline Jay  Indonesia
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Hi thanks Bob. BTW, did you notice the fatiguing characteristic of the amp? It's the sound of mosfet in class-B I'm very familiar with. Having been working with discrete mosfet amps for years, I'm very pessimistic.

I have been around one hour now listening to the amp with Daniel's component setup (underrated 20V rail) and already felt depressed. Very fatiguing.
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Old 28th December 2012, 04:31 PM   #42
bcmbob is offline bcmbob  United States
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Jay, are you using the same board as in the picture?
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Old 28th December 2012, 07:23 PM   #43
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by bcmbob View Post
Jay, are you using the same board as in the picture?
No, mine is smaller. FB resistor direct between pins. 100n MKT bypass caps standing on the pins (+/-Vs to GND) on the trace side.

Like Daniel I used 2uF (MKP Sprague) between rails. Imo this is too big. When long time ago I tweaked the cap by ears, I found the value to be less than 1uF. Smaller size so can be mounted closer and direct between the +V and -V pins.

The on-board 470uF (also following Daniel) is I think too small (off-board I have 8x 4700uF Roederstein). I prefer 1000uF (many caps of this size ).

The choice of component values, such as the FB resistors, are just strange
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Old 29th December 2012, 04:18 AM   #44
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Quote:
Originally Posted by Jay View Post
Hi thanks Bob. BTW, did you notice the fatiguing characteristic of the amp? It's the sound of mosfet in class-B I'm very familiar with. Having been working with discrete mosfet amps for years, I'm very pessimistic. I have been around one hour now listening to the amp with Daniel's component setup (underrated 20V rail) and already felt depressed. Very fatiguing.
Well, that's very strange. My own TDA7294 is very hard to tell apart from the LM1875 projects, or my PA amp, none of which are either depressing or fatiguing. An unoptimized TDA7294 should sound about as good as an STK, OR a TDA7294 done well should do rather transparent high fidelity. If it isn't within that approximate range of performance, then there's an error.

Trying to guess what could be the problem:

DC offset and/or fake chip cooked a tweeter? (needs speaker protection)
Bad quality NFB cap used? (try paralleling a tiny bypass cap)
Input cap was larger than 1u? (try 0.68u)
Bad/overlarge amplifier board power caps? (try replacement 220u)
Very dulling power supply? (try the diode trick)
Speaker negative sent to the power board? (try the amp board 0v)
Bias fail from under-volting? (try 35+35VDC power)
Printed circuit board layout issues? (replace or omit)
Accidental tone control installed somewhere? (debug)

Did you try this?:
Click the image to open in full size.
Experiment: Perhaps the lack of gain causes the source to strain (source not useful in that condition), so try feedback resistor at 100K to max out the gain and put the input load to 56K--slightly worse for the amp, but possibly help the source a lot.
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Last edited by danielwritesbac; 29th December 2012 at 04:31 AM.
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Old 29th December 2012, 04:40 AM   #45
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Quote:
Originally Posted by Jay View Post
The on-board 470uF (also following Daniel) is I think too small. . .
Following me? Nope. You didn't. But, that value is okay for LM1875's amplifier board power caps.
Quote:
Originally Posted by danielwritesbac View Post
I think you're going to need to upgrade to some 220u caps. . .
TDA7294 is not a gainclone. The bigger the amplifier board Power circuit capacitance, the duller TDA7294 will go (similar to some discrete amplifiers). I think we should use only as much as necessary to prevent shout. For your amplifier board power caps (range 100u through 330u--likely values are 220u or 270u), it may take some experimentation to find a perfect value, because PCB layout does affect sound.
Quote:
Originally Posted by Jay View Post
The choice of component values, such as the FB resistors, are just strange
True! However, to get extra low bass from TDA7294, we have to use audiometric values just like a discrete amplifier; and then, feedback-shunt of 680R with 1000u has a problematic discharge failure mode because it is just too big. Therefore, the feedback-shunt and feedback resistor values have to go higher which conveniently allows using a smaller cap. Alternative: If you really do want 680R, then you need a DC tracker.
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Last edited by danielwritesbac; 29th December 2012 at 05:04 AM.
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Old 29th December 2012, 07:40 AM   #46
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Quote:
Originally Posted by Jay View Post
(off-board I have 8x 4700uF Roederstein). I prefer 1000uF (many caps of this size ).
Attached is a power supply in use with 200va transformers for TDA7294 Monobloc amplifiers (each chip has its own transformer and power board).
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File Type: gif CTPowerBasic.gif (5.8 KB, 742 views)
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Old 29th December 2012, 09:57 AM   #47
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by danielwritesbac View Post
Following me? Nope. You didn't.
Quote:
Originally Posted by danielwritesbac View Post
TDA7294 non-inverting amp:
Pair 220u amplifier board power caps and a single 2u polyester rail to rail.
47u (or 68u) for bootstrap
10k for input load resistor
10k for mute (no cap)
22k with 10u timer for standby
60k (120k||120k) for feedback resistor
2.7k for feedback-shunt resistor
220u (or more) for NFB cap
1u (or less!) for input
Output zobel is 10n with ~8R
Add RF filtering to input cap
Use a stiff power supply (20,000u per rail, 3a per chip)
Use less voltage than max (under-volt)
Oops... I thought you meant 2x220uf per rail. As I have space for one pair on pcb so I took 470uF. Will try 2x220...

Quote:
Originally Posted by danielwritesbac View Post
TDA7294 is not a gainclone. The bigger the amplifier board Power circuit capacitance, the duller TDA7294 will go (similar to some discrete amplifiers). I think we should use only as much as necessary to prevent shout.
To my ears, with 470uF the amp already shouts. I can hear that the problem (distortion) is with female vocal (shouting).
Quote:
Originally Posted by danielwritesbac View Post
Well, that's very strange. My own TDA7294 is very hard to tell apart from the LM1875 projects, or my PA amp, none of which are either depressing or fatiguing. An unoptimized TDA7294 should sound about as good as an STK
Well, this is the reason why I'm building your version (and I have built many versions). To prove to myself that I know exactly what I hear.

Quote:
Originally Posted by danielwritesbac View Post
Trying to guess what could be the problem:

DC offset and/or fake chip cooked a tweeter? (needs speaker protection)
Bad quality NFB cap used? (try paralleling a tiny bypass cap)
Input cap was larger than 1u? (try 0.68u)
Bad/overlarge amplifier board power caps? (try replacement 220u)
Very dulling power supply? (try the diode trick)
Speaker negative sent to the power board? (try the amp board 0v)
Bias fail from under-volting? (try 35+35VDC power)
Printed circuit board layout issues? (replace or omit)
Accidental tone control installed somewhere? (debug)
No offset, very silent. Used 2 speakers. Main one is for testing amplifiers: a full-range driver (no crossover, no tweeter).

For FB cap I used Nippon Chemicon SM-D Bipolar 2x100uF. Will replace one with 47n-100n, whatever fits on the pcb.

Input cap was 1uF (fit well on PCB). Will try lower values. I think a WIMA MKP will fit.

On-board 470uF will be replaced. Maybe with Philips 220uF.

Very dulling power supply? I think the amp is not dull, rather, it tries to mimic other sweet sounding chip amps, which is not accurate imo (distortion with vocal). My power supply modules are for testing new amps. I have used 8x3300uF Nichicon, same value with your 12x2200uF. I used 2x4 hyperfast diodes. No bleeder.

Speaker ground is from PCB (as designed).

Yes, I agree underrated voltage (20V) is a problem. I think optimum is around 31V, as datasheet example to measure performance. Will use 28V as some on-board capacitors are rated 35V only (to fit on small pcb).

PCB is already good (have been used for many good sounding TDA7294 before). And I made it better with PTP and using solid copper wire along the high current track.

Quote:
Originally Posted by danielwritesbac View Post
Experiment: Perhaps the lack of gain causes the source to strain (source not useful in that condition), so try feedback resistor at 100K to max out the gain and put the input load to 56K--slightly worse for the amp, but possibly help the source a lot.
Hmmm... I'm against such approach. What I'm doing here is trying to listen to TDA7294 setup that others are listening, so I can understand their comments about the sound. And BTW, I have always had benchmark amps next to the amp I'm playing with
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Old 29th December 2012, 01:37 PM   #48
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by Jay View Post
Oops... I thought you meant 2x220uf per rail. As I have space for one pair on pcb so I took 470uF. Will try 2x220...
So I tried 2x220uF. I couldn't find my Black Gate. My options were Philips, Cerrafine, Panasonic Pureism (which I don't like) and Panasonic CE HF. The Panasonic is twice the size of Cerrafine (and I think only slightly smaller than BG) so I used that.

I'm afraid I was listening to different brand instead of different capacitance here But I also change the bootstrap cap from 2x33uF Elna to Philips KO (that I like) 68uF.

Didn't want to change too many things at once. But alas, I changed the location of the main speaker

The first moment I heard the sound, I didn't recognize the sound of TDA7294 I have been familiar with. With the last change the sound is less gorgeous but more musical. I like the latest setup with 2x220uF, whatever causing it

But still, ehm, it's a chip amp
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Old 29th December 2012, 02:17 PM   #49
Jay is offline Jay  Indonesia
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I don't know why, I feel disturbed with the sound of this amp... But not by my other TDA7294 amps... Even with the same power supply...
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Old 29th December 2012, 02:52 PM   #50
bcmbob is offline bcmbob  United States
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Jay, You are miles ahead of me ( I'm still working on some graphics to keep things defined for we less experienced) but I did find some 220 U caps to try. Unfortunately, they are all large amd previously used. Is simple off-board mounting OK, or should I attempt to extend the leads ? (I've done that before successfully)

I'm working on the dual transformer setup right now.
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