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iggy111 29th April 2013 11:07 PM

"Bloated" sound from GC?
Hi all,
after comparing my GC with quality commercial amps I've confirmed that my build sounds "bloated". By that I mean that there is more than enough bass and mid-bass, but mids and highs seem recessed. The amp obviously has more dynamics compared to others, lacks in HF transparency, but it sounds "bloated".

I'm just wondering if anyone else had a problem like this, before I test it without the regs and play around to see if I should change some components?

These are my changes to a stock chipamp:
Removed R1 and Ci
Lowered R2 to 10k to reduce DC offset
Increased Rz to 5R
Added 220pf across pins 9 and 10
Added carlosfm regulators and snubbers
I don't use a decoupling input cap at all

Trafo is a toroid 160VA, 2x29V sec, regs are fine tuned to 30V

djoffe 30th April 2013 12:00 AM

can you post a schematic?

iggy111 30th April 2013 11:02 AM

Ok, here is the original amp scheme with my changes:

And here is a link to the regulated snubberized PSU:

Can you guys comment on the overall sound of your GC's?
I read somewhere that "a GC is known to lack authority in the bass" (compared to a F5, I think), which is exactly the opposite of what I'm experiencing. I guess that HF transparency will be achieved with better quality caps, like Silmic II, but for now I just want to understand if there is something wrong here.

djoffe 30th April 2013 12:24 PM

Please clarify "Removed Ci". Does that mean there's nothing there...e.g. an open..., or a short circuit in place of it?

AndrewT 30th April 2013 12:47 PM

Go back and make this an AC coupled amplifier.
Add in all the "optional" components listed by National

Ensure the HF decoupling has a very short route. The two ceramic caps should be soldered to each other. The other ends get soldered to the power pins.
The MF decoupling can be a little further away. BUT and this is very important: The link between the two MF decoupling caps MUST be very close to the junction between the two HF decoupler caps. Connect these two points together to create your Power Ground.
Connect the output Zobel to this Power Ground.
This minimises the route that the HF spikes must follow around the output stage. This really is very important and 99% of the PCB layouts (discrete and chipamp) that get posted on this Forum completely ignore this HF route requirement.

Ensure the power input leads and traces and the Power Ground all follow each other very closely until they reach the power pins. Again this is ignored in almost all posted layouts.

Check the bandwidth of your two input filters. Are the F-3dB and F-1dB frequencies sensible to the audio signal you want to pass?

AndrewT 30th April 2013 12:49 PM

When you have done all that is in post5 and listened to what a properly implemented chipamp can sound like, then you are in a position to build further implementations to try to find improvements on the basic AC coupled, fully componentised and filtered chipamp.

iggy111 30th April 2013 03:09 PM

@djoffe - yes, I removed the Ci completely after a lot of reading here on the forum. That's actually an option with a Chipamp, you just need to resolder the R3 to another hole on PCB. I wanted minimalism, especially after reading about Peter Daniel's 3875 based amp.
Here is his scheme:

@AndrewT - by now I know you are very sensitive to "improperly implemented" stuff, but would you please comment on PD's minimalistic approach then? I hear only praises about it. I think you are right, and I will try to properly implement the chipamp one of these days, but you are aware that using coupling caps and Ci means I have to buy Mundorfs or such - any cheap cap (even Panasonic FC here) sounds bad, and I'm much happier with a "no cap" right now.

Will post a photo of my build soon, it will be easier to comment then...

AndrewT 30th April 2013 03:37 PM

I have been castigated for interfering (commenting) in Peter's/Audiosector's Threads.
I pointed out that some of Peter's philosophy should never be followed by beginners.
I pointed out that DC coupled increases the risk of damaged speakers.
I pointed out that Peter's implementation was developed in conjunction with very efficient speakers and thus could not be transferred directly across to normal efficiency, nor low efficiency speakers.
I made various other comments (too long ago to remember) that tried to forewarn Members of the dangers in Peter's advice.
If you really are interested you could search out my posts.

I do not believe that a properly sized NFB capacitor that imposes no AC signal on the signal current can add to the distortion of an amplifier.
That alone allows one to use a "normal" commercial quality capacitor and NOT be able to hear added distortion compared to an expensive capacitor.

If the expensive capacitor does "sound" different then it is faulty, or it is the wrong size.

If you want a DC coupled amplifier then my advice is learn how to protect your speakers. And learn why DC detection and DC servo and speaker isolation and maybe input muting, should all be implemented. That is a lot of learning and a lot of board space and a lot of cost to avoid AC coupling that can give very good sound.

iggy111 30th April 2013 05:13 PM

Ok, a few questions:
What happens if you hook up an amp like PD's to a normal or low efficiency speakers? How does that sound or what problems come up?
And to return to the topic - could that be the problem with my amp? My speakers are 89dB (Tannoy Eyris 3).

About speaker protection - I have a pair of muting relays connected to a switch on the front panel, so every time I turn the amp on, it powers up muted, so then I unmute it. And before powering down, I mute it first. Absolutely no pops or clicks, I know I could do it automatically but manually is fine with me. Of course, I mute it first before powering the cd-player or PC on or off, so I think I won't get the DC surge (is that the word?) amplified through to my speakers.

PS could you post a schematic and/or a photo of all the things you mentioned in post #5? I can't figure out what you mean by soldering the ceramic caps together first, because they are each on a different amp board.
And also how do I check the bandwidth of my input filters? Are you refering to the coupling caps or Ci + R3?

AndrewT 30th April 2013 05:20 PM

So many questions I do not have time to detail all you ask for.

A pair of radial 0.1" xr7 100nF 100V caps make good HF decoupling.
Cut a lead on each very short. Solder these short ends together.
Cut the other two leads to about half length. This soldered junction is a Power Ground
Solder one half length lead to +ve pin.
Solder the other half length lead to the -ve pin.
You now have the power pins connected to an HF power ground.
Keep going, build an MF Power Ground using electrolytics.

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