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Old 6th April 2014, 11:19 AM   #1
beanbag is offline beanbag  United States
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Default Some questions on an Adcom 5400

Hello,

I hope this is the right forum to ask these question.

I recently got myself a used Adcom GFA 5400. It is the Version 1 made in Japan. As compared to my other Adcom 535L, it seems that the higher notes that a cello plays have kind of a nasally, harsh, scratchy, annoying sound. The highs sound a little bit more forward and harsh, vs the 535L sounds more smooth or laid back. These differences seem small to me, and I am not even sure they exist. So a couple of questions:

1) I read about the main difference btw bipolar and MOSFET amps is their distortion characteristics. What would be the most useful test for me to measure on the 5400 to see this (or rather, to differentiate its sound from the 535L)?

2) I would be willing to change out the caps or make a few other tweaks if I am pretty sure they would make a difference. So for example:
a) the power supply caps don't really look bulged, but the top are convex by about 1/16" of an inch. Worth changing out?
b) Any other caps worth changing out?

And what kind of measurement to make to confirm that these changes made a difference?

3) I would also like to cut down the gain by about 10 dB since I am driving it with a DAC directly. Where would be a good place to do it?

4) I read another thread where some guy kept increasing the bias to ridiculous levels, and claimed the sound kept getting better and better. Now the service manual says 35mV over .56 ohms, or 63mA. I briefly increased it to 180mA, but didn't really notice a big difference. I didn't want to toast the amp or anything. Is this worth doing?

I don't know if I am allowed, but I can post a schematic if requested.

Thanks
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Old 13th April 2014, 06:32 PM   #2
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Short version why 5400 doesn't live up to its potential:
3 pairs of output transistors and source resistors not matched well enough.
PS too small w too much ripple.
HS too small for glorious amounts of bias.
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Old 18th April 2014, 06:18 AM   #3
beanbag is offline beanbag  United States
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I should read more articles about the Pass or MOSFET designs, but what is it about them that they need high bias to sound good? Or rather, that higher bias makes them sound better. If it's because it reduces the distortion, then why not design in lower distortion or higher feedback in the first place?
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Old 18th April 2014, 11:36 AM   #4
flg is offline flg  United States
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Quote:
Originally Posted by beanbag View Post
I should read more articles about the Pass or MOSFET designs, but what is it about them that they need high bias to sound good? Or rather, that higher bias makes them sound better. If it's because it reduces the distortion, then why not design in lower distortion or higher feedback in the first place?
Yes, reading more of Pass will answer these questions. Even the classic Zen series of amp articles probably covers this subject well.
Basically, you can't just "design in" lower distortion. You do the best you can, with the devices that are available, at balancing all the necessary parameters to acheive the design goals. Most device parameters vary with variation of other parameters as can be seen in the curves on the datasheet. But, operating in a linear, low distortion manor is sometimes difficult. That might be graphed more as a strait line rather than a curve. You can see from some curves in the datasheet that as current goes up, the curve becomes more of a strait line.
Feedback is another touchy issue with some. Many think that although feedback lowers the measured THD numbers, the actuall residual distortion products of a high feedback circuit, although low, are more damaging to the sound than the simpler, lower order distorion of a circuit with much less or no feedback.
If you've been a member here for 10 years I would have thought you would have picked this up? You must of spent to much time in one of those other forum catagories. Passdiy and firstwatt www sites have many short papers and Zen articles that will get you up to speed on the Pass philosophy
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Last edited by flg; 18th April 2014 at 11:41 AM.
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Old 18th April 2014, 11:40 AM   #5
flg is offline flg  United States
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Quote:
Originally Posted by Andersonix View Post
Short version why 5400 doesn't live up to its potential:
3 pairs of output transistors and source resistors not matched well enough.
PS too small w too much ripple.
HS too small for glorious amounts of bias.
Yep, that's most of it!
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Old 18th April 2014, 05:01 PM   #6
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NP's 'Leaving Class A' article is a good summary.
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Old 18th April 2014, 06:48 PM   #7
delecoy is offline delecoy  United States
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I'm sure there are plenty here that know better than me, and would probably even call bs- but I could hear when the bias was out on my gfa-535 ( high or low ) and it was always most evident in the treble. The original pots were finicky to set, and constantly drifting. Replaced them to alleviate the problem. Anything outside +-25% of the recommended bias setting always had me opening it for a reset. Having read so many threads here on the relationship between bias and sound quality had me wanting to set it higher, but my ears were telling me otherwise with those amps. Perhaps the 5400 is behaving similarly.
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Old 18th April 2014, 06:55 PM   #8
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GFA-535 is a BJT amp, so it has an 'optimum' bias point. But MOSFET amps like the GFA-5400 sound better and better (lower distortion) with more bias.
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