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Please technically explain this audible mod
Please technically explain this audible mod
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Old 28th June 2017, 01:13 PM   #11
HotIce is offline HotIce  United States
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That schematic has neither global feedback, nor a DC servo, and it can have some gain, so, without input decoupling capacitors, any DC component in input will be amplified (if the 680 Ohm resistor is wired, up to ~8 times) and set to the headphones.
I personally do not believe that any decent capacitor (that schematic, as component choice, reveals someone who has thought about quality - assuming parts are OEM and not clones) when made work within its limits (and especially in the input, with low/to-zero DC component across it), is going to affect sound quality.
This is a good read, in case you have no already:

Capacitor Characteristics

And this:

Blog - How Much Distortion Can We Hear With Music? | Axiom Audio
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Old 28th June 2017, 04:37 PM   #12
DF96 is offline DF96  England
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Originally Posted by HotIce
that schematic, as component choice, reveals someone who has thought about quality - assuming parts are OEM and not clones
It may reveal someone who believes that 'quality' is found in particular component brands. It may reveal someone who believes that such brands are more important than getting the circuit right. Or maybe it just reveals what he assumes about the knowledge of his customers and reviewers. Sending DC to headphones is unlikely to improve the sound.
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Old 29th June 2017, 12:54 AM   #13
xrk971 is online now xrk971  United States
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Please technically explain this audible mod

That's a lot of super glowing reviews for what appears to be simply an opamp with a BC550/560 and BD139/140 output stage?

Where is the innovation in this?
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Old 29th June 2017, 01:32 AM   #14
abraxalito is offline abraxalito  United Kingdom
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Originally Posted by elmura View Post
The purpose of the mod, as with any I do, is to attempt to improve the sound quality. This amp has great tonality but was missing clarity - a bit veiled.
As the schematic is a Lehmann (as xrk is pointing out) its very close indeed to one I had a play with a while back, wrote it up on my blog - http://www.diyaudio.com/forums/blogs...phone-amp.html

The TL,DR version is the poor dynamics turned out to be a function of the opamp's loading, poor OPS PSRR and also poor grounding. No changes to caps in the signal path were needed for satisfying sound.

So, armed with this measurement and the following schematic of a replica, do you still think my mod has caused a problem?
To get a handle on the noise currents flowing in your grounds, the layout is orders of magnitude more useful than the schematic.
I know you think you understand what you thought I said but I'm not sure you realize that what you heard is not what I meant - Alan Greenspan
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Old 13th July 2017, 05:24 AM   #15
elmura is offline elmura  Australia
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Sorry for the long delay to provide feedback. A computer failure, and 3 weeks illness to blame.

Based on comments & some analysis, I rerouted the 4 signal wires carefully; desoldered the original wires from the board (previously I only desoldered them from the RCA sockets); and desoldered the pre out wires in case of interference from torroidal transformer or external noise.

Glad to report that bass is juicy again, and soundstage has improved. But I'm still finding depth of stage not as great.

So my question is: do you think that the designers choice of joining the signal grounds together at the end of the board (closest to the RCAs) was causing more crosstalk, and thus deepening the stage?

I recall that prior to the mod, the stage was deeper, but the sound was more distant, less clear, less present.

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Old 13th July 2017, 06:09 AM   #16
rjm is offline rjm  Japan
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Please technically explain this audible mod
Originally Posted by elmura View Post
Click the image to open in full size.
Ye basic noninverting op amp plus open loop diamond buffer affair.

C103,C104 are completely optional. C101,C102 are for safety's sake but if you know the input has no DC offset you can short them out it won't affect the electrical performance of the circuit.

Looking at the rest of the circuit though, I'd have to say that those capacitors are the least of the problems. Ugh. Even the position of the caps is wrong, they should be in series with R13,14 with a 47k resistor added in parallel with C105,106. As it stands the output DC offset will vary with the position of the volume control... a problem they try to alleviate by making R13,14 so large, which adds to the circuit noise. The op amp input impedances are not balanced properly either at DC or audio bandwidth.

Dual op amp - yuck. Dual op amp powered off RC filtered rails - double yuck.

And that diamond buffer. R26,27 and R30,31 are added to goose the output bias current, but they kill the PSRR of the buffer. Also 1.5k for R25,28,29,32 would be better off replaced with current sources for lower distortion, or at the very least increased.
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Old 13th July 2017, 11:24 AM   #17
mfratus is offline mfratus  United States
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I would put those caps back in, and remove C105 and C106 or reduce their values to maybe 47pF. They shunt highs to ground at the input of the op amp. C103 and C104 are "sweetening" caps, attempting to pass more extreme highs. I would reduce R19 and R20 to 10 Ohms to firm up the supply. Yes, you could be getting DC from your source, and you could be getting extreme lows that are interfering with what you can actually hear.
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Old 13th July 2017, 01:16 PM   #18
elmura is offline elmura  Australia
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I just measured DC at source and output of the amp:
DAC: <0.2mV each channel
Amp: 13mV and 9mV

Also, as I have selected unity gain on the switches, volume on the pot is typically 12:00 to Max pending music source. So, what would the effect of removing R13/14 and reducing C105/106 values?

For reference, I've attached a pic of the internals with the current mod. The grey wires are the desoldered Pre Outs. The 4 wires are source and are positioned adjacent to the original PCB traces & soldered direct to the Pot
Attached Images
File Type: jpg 2017-07-13 23.04.01.jpg (226.9 KB, 265 views)

Last edited by elmura; 13th July 2017 at 01:43 PM.
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Old 13th July 2017, 01:29 PM   #19
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Without global NFB, the thermal inaccuracies or runaway of the output stage is uncompensated, and very risky for the earphones live.
Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001.
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Old 13th July 2017, 02:20 PM   #20
mfratus is offline mfratus  United States
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The Low Pass filter formed by the R13 & C103 (10K and 100pF) is significant. You can calculate the frequency. R13 is there probably to minimize the change as the pot is moved through its range.
The input circuit does leave much to be desired. The chip datasheet says, "To maintain low distortion, match unbalanced source impedance with the appropriate values in the feedback network as shown in Figure 28." Other's comments were right, using a variable resistor in the input leads to more distortion.
Instead of just removing stuff (it was in there for some reason) I would tweak the values. This is design work.
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