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Some Thoughts on I/O of Amp6 Kit
Some Thoughts on I/O of Amp6 Kit
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Old 31st October 2008, 01:52 AM   #1
CLS is offline CLS  Taiwan
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Default Some Thoughts on I/O of Amp6 Kit

I got my Amp6 kits the other day and yet to have time to assemble them. Before that, I got some ideas.

First of all, I'd like to use these amps for the bass channels in my active-xover system, in which there're 2 woofers per side. So I bought 2 * Amp6 kits, total 4-ch amp to serve 4 woofers.

1. Replace the input cap by transfomer:
Click the image to open in full size.

The input bias voltage can be isolated by the transfomer. While I'll use both "amp-channel" in the same "system-channel", so it doesn't matter to tie them together. Exchanging the output phase related to speaker driver solve the problem of 'differential' input.

2. How about the output chokes? I saw them individually put on each output leg. Can I replace them with "differential mode" choke?
Click the image to open in full size.

How will they work?

I'm new to class-D world, any comments are welcome and appreciated.
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Old 31st October 2008, 04:37 AM   #2
dweekie is offline dweekie  United States
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Which input transformer did you have in mind? I haven't seen many that seemed cost effective for this role.
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Old 31st October 2008, 06:24 AM   #3
CLS is offline CLS  Taiwan
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It's not really for cost saving. I've experienced the direct comparison of cap- and DC-couple in line level signal. I don't like caps (from no-name to those big name ones).

And, I got 2 Lundahl LL1582 left unused in the drawer for quite some time. I guess they should be Ok for this usage.
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Old 31st October 2008, 07:17 PM   #4
Pano is offline Pano  United States
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Some Thoughts on I/O of Amp6 Kit

I've been waiting for years for someone to post the transformer input scheme. Well done!

Yes, it works, and works well. And for your application, it is just fine. Sometimes you have to tie the center tap to 2.5V to prevent hiss.

As for the common mode output chokes - i don't know. I did try it years ago, but can not remember the results. I think that Brian tried it too, maybe he can give you more info.

IIRC, not all the noise is common mode, so it may not work well.
Take the Speaker Voltage Test!
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Old 1st November 2008, 12:03 AM   #5
CLS is offline CLS  Taiwan
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Thanks for the tip about the center tap

As to the output choke, I believe the one I drew above is not common mode choke -- look at those dots.

The output stage of TA2020 is bridged internally. So the switching noise should be differential mode (opposite phase) -- well, if everything is perfect, which may not be the case mostly.

OTOH, I remember reading somewhere that one can raise the satuation current by winding 2 coils into one core (same physical construcion as CM choke), but I just can't remember the details....
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Old 1st November 2008, 02:36 PM   #6
Buckapound is offline Buckapound  United States
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Outside of digging in the junk drawer for some NOS Lundahls (must be some junk drawer!), can anyone suggest a part number from Mouser or Digi-Key for some suitable input transformers?Would something like MT4151-ND (Digi-Key) be suitably hi-fi? I'm having ground loop issues when connected to a laptop and this would be he best way to fix, I think. Not to mention dumping the caps.

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Old 1st November 2008, 04:55 PM   #7
BWRX is offline BWRX  United States
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Your input schematic is incorrect. You show the transformer secondary going to both inputs. One side should be grounded or connected to the bias pin. You will still need coupling caps for each input if you ground one side of the secondary because the input nodes are biased at 2.5V - you don't really want a DC voltage across the secondary of the input transformer. If you connect one side to the bias pin you wouldn't need to use coupling caps.

Your polarity dots on the linked output coils are incorrect. You want them to be common mode so they pass the differential output current. Like panomaniac, I did try that a while ago (there's a thread here somewhere) and honestly couldn't hear a difference between two separate coils and two coils on one core.
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Old 2nd November 2008, 01:18 AM   #8
CLS is offline CLS  Taiwan
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Thanks a lot.

So, I guess the I got the following chances:

1. basically the diagram above, but tie the CT of secondary to the bias pin (pin14).

2. tie on end of the secondary to the bias pin, and the other end to both inputs.

How would they work?

Yesterday I finished one board (but not entire amp with power, I/O etc.... ). Geez those parts are tiny!

I've been a tube guy and not familiar with such small parts

Not tested it yet...
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Old 24th May 2010, 02:48 AM   #9
CLS is offline CLS  Taiwan
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This is a long long overdue update

Fact is, my first Amp6 kit assembly failed (power supply OK, voltage correct, warm regulator and chip, but no signal I/O... checked, resoldered again and again, still nothing. ) So frustrated that I didn't want to try the other.

Until last couple of weeks. I managed to assemble the other kit on hand. This one was smoothly done without any drama, sounded good at the first try. How lucky

Then I started to fiddle it with several tricks - a not-so-successful current feedback mod (with small OB trial), then, now about the I/O setups as the topic.

First about the sounds. I believe many people are very familiar with Amp6 already. This little thing gives a very good first impression - a lively, energetic sound with excellent details and large sound field. Many little sounds (or noises) floating in the background are kind of emphasized (or magnified) to give an impression of "hearing new things in records".

After the small OB mockup, I hooked it up to my main system to drive the mid-high section (200Hz and up). I found 2 major drawbacks: 1) the hiss, 2) slightly edgy HF

Including the originally equipped electrolyte cap, I've tried several other types. TBH, except that 10uF one causing too much switch-on thump, their differences in sonic are almost nothing to talk about. (Ok, call me deaf. I really don't care about that kind of differences.)

Then I tried added common mode chokes after the output. It surprised me that it only takes several loops to make relatively high inductance -- in mH order! (That's strange, the originally equipped chokes take tens of loops to make uH. Hmm.... ) At first I was worried the inductance would be too much to affect audio band, but WTH, they are common mode chokes, should be no effect to the differential mode bridged output signals.

Results are almost no or very very little effect in hiss and the sound I don't understand. The hiss is actually inaudible beyond 0.5m, so it's not bothering me very much. My major goal is to 'fix' the slightly edgy HF.

So, finally, the trafo-coupled input. My previous experiences on bad impressions of coupling caps are mainly in the bass. DC or trafo-coupled circuits give much better bass response than caps -- clearer definition, more solid. However, now this amp is serving mid-high, no bass, oh well....

The transformer is LL1582 (as mentioned), which is decent for line level (if not excellent). In my previous usage, the impression was smooth and a tad of loss in detail, no big deal though.

Here I have voltage gain more than enough, so I cofigured the trafo in 2:1 step down. Wired one leg of the secondary to the bias pin. It worked well, without any slightest hint of additional noise or any other abnormal condition.

But again, I couldn't detect any meaningful sonic differences (against those caps). Argh ~ WTF, am I really deaf ?!

The slightly edgy HF is still there. I just couldn't get rid of it. It appears on almost every HF overtones - throaty vocals, breaths, strings. It's more obvious and annoying in these continuous sounds. In contrast, at those transient sounds, although a tad of emphasized, adding some more 'sparkle' and sharpness, are relatively welcome.

The HF overtones of the real things (in a proper space and distance) are usually not so "pronounced". Instead, they are very smoothly extended to almost endless, with a very delicate and dense quality. The edgy HF of this amp seems magnify a particular portion of the whole texture, causing an effect of black-lined-contour. Although the line is very fine, it's there.

In contrast, my old 6S45 spud amp renders the overtones in a more natural way without any shortage of details.

So, Amp6 can not replace the spud amp. Maybe it's because I haven't revealed its full potential, or maybe I'll never will (it's all done in the chips?). Kind of frustrated

BTW, the source is modded Behringer DEQ2496 with transformer-coupled output (skip all analong stages in the box), and the pre is DIY transformer-loaded 417A. Speaker is DIY Oris 150 midhorn and 2 waveguided tweeters per side, 105~107dB/w above 200Hz.
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