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Please technically explain this audible mod
Please technically explain this audible mod
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Old 27th June 2017, 01:09 AM   #1
elmura is offline elmura  Australia
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Default Please technically explain this audible mod

Hey guys. I've recently acquired a used Lehmann Black Cube Linear headamp to use with my Sennheiser HD800 & Matrix X-Sabre DAC.
After a couple weeks listen which I enjoyed but found it short on clarity & a little distant, I opened her up. Along with the schematic, I removed 2 input capacitors on each channel (a 22nF & 1.5uF in parallel).
I then ran elcheapo 20AWG wires from the input RCA direct to the Alps RK 27 PCB pins. The leads run under the PCB the length of the chassis.

Here's the question: The sound is now clearer as expected, but the soundstage has widened too much and I've lost stage depth. The bass richness & impact seems to have lessened too.
I'm puzzled Please technically explain this audible mod. Some music is less enjoyable, some more.

Can you explain why?

Sent from my LG-D855 using Tapatalk

Last edited by elmura; 13th July 2017 at 02:12 PM.
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Old 27th June 2017, 01:52 AM   #2
Hikari1 is offline Hikari1  United States
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You are hearing the way the source actually is presenting it, or more so than with the capacitors

The capacitors have some distortion and non-linearness which place emphasis on certain frequencies which you perceive as bass or depth based on what you are listening to. Also, there is a frequency cut off presented with the capacitors in place.

But if your dac is outputting D.C. (The capacitors are there to block D.C. from the source) you may be hearing something else which may be not good. D.C. on the output the tour headphones or the input ic struggling with the D.C.

Also, your wire choice may influence the sound. Perhaps more than the caps.

Some people like warm and romantic. The sound of a cap may be preferred but it is farther from what the source is playing.
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Old 27th June 2017, 01:58 AM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by Hikari1 View Post
You are hearing the way the source actually is presenting it, or more so than with the capacitors
Yes, and the volume control may also affect the sound as much.
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Old 27th June 2017, 02:43 AM   #4
Hikari1 is offline Hikari1  United States
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Also, people like what they are used to. Almost always. Even if different is "better".
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Old 27th June 2017, 03:44 AM   #5
abraxalito is offline abraxalito  United Kingdom
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Loss of soundstage depth suggests common ground impedance coupling. Or noisy ground at your volume pot in other words. Does the depth diminish even more at lower volumes?
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Old 27th June 2017, 01:13 PM   #6
elmura is offline elmura  Australia
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Food for thought so far. Thanks. I'm going to take some measurements & do some more listening.

A thought: In original setup, the RCA inputs fed a common ground at the end of the PCB which then ran a single trace to the front POT. Many ground VIAs along the way. In my setup, I ran 2 separate ground wires direct to separate POT pins, bypassing the trace. Ground loop?
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Old 28th June 2017, 01:25 AM   #7
HotIce is offline HotIce  United States
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Usually designers don't put DC input decoupling for nothing, so removing them is not a good idea.
If you remove them, you DC couple the output of the previous stage, with the input of your amplifier, whose inner input might not like at all to be at ground level.
As a trivial example, take this utter simple, one BJT amplifier:

Click the image to open in full size.

If you bypass C1, you wire the base of the BJT to 0V, and it will be distorting as hell.
Or, if for any reason there is some DC component in input, the bias of the BJT will be totally off.
Some other amplifiers might even go in smoke.
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Old 28th June 2017, 09:39 AM   #8
DF96 is offline DF96  England
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Quote:
Originally Posted by Hikari1
Also, your wire choice may influence the sound. Perhaps more than the caps.
No, much less than the caps. Unless the problem is that an unshielded wire was used from the RCA so it picks up hum and interference.

The input caps block DC, so keep your amp safe. They add a high pass filter, to block very low frequencies. The fact that the main input cap is bypassed may add a high frequency notch.

Changing the grounding may have made a difference, possibly a bigger difference than omitting the caps. It all depends on the details of the circuit.

Why change the circuit? Did you believe that the original designer got it wrong?
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Old 28th June 2017, 11:50 AM   #9
elmura is offline elmura  Australia
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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. Upon research of the schematic, seeing caps in the signal path, which are often audible, was the reason for doing so. The DAC feeding it was the flagship model from Matrix so I didn't figure it would need DC blocking.

Before I did the mod, I took some measurements with RMAA. I've been using this for years whenever I want to examine a new piece of audio electronics. The ADC is a E-MU 0204 hooked up to a 2nd PC. The source is my desktop playing from Foobar in WASAPI mode with Windows Mixer Enhancements disabled. After initial testing of the amp alone, final tests are done with the headphones connected in parallel, with the ear pads mounted flush on a headphone stand to replicate the electrical & acoustic impedance of the phones being on my head. Why? To simulate real world usage.

As suggestions are indicating that the mod may be a bad thing, just now, I completed the same test routine with the modified Lehmann amp to examine what may be happening.

Both good and bad news.

Good new, no negative changes & slight improvement. Noise lowered 0.9dB; 1kHz THD identical; IMD figures identical but there is one strange anomaly at 300Hz in the graph with the mod; THD Swept Tones showed approx 1dB full better with the mod.

Bad news: Nothing unusual shown which doesn't help.

So, armed with this measurement and the following schematic of a replica, do you still think my mod has caused a problem?
Click the image to open in full size.

PS - I know the free version of RMAA limits what you can see and the software doesn't get much development but I'm very familiar with it and have made myself a library of test results. If you know of a better app, I'm all ears.
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Old 28th June 2017, 12:54 PM   #10
DF96 is offline DF96  England
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MKT is polyester dielectric, so very slightly nonlinear. Perhaps not the best choice. However, signal levels will be fairly low and the LF rolloff is at 2Hz so there will not be much signal across the cap from 20Hz up. I assume this cap is fairly large so that might explain the noise when in circuit, due to stray capacitance.

The rest of the circuit is just a fairly standard augmented opamp. I note that there is no output coupling capacitor so that endangers the headphones in the event of a circuit fault. It also means that the input coupling cap is the only LF rolloff.

If it was my amp and I wanted to mod it I think I might try replacing the input caps with something like a 0.22uF or 0.47uF polypropylene, and think about adding an output cap.
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