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Old 15th March 2019, 08:45 PM   #111
prasi is offline prasi  India
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ESP HPA
it is very very quiet.
I dont know why its giving you troubles!
I think xrk measured the esp hpa and shared the results with me.
It was quiet.
Since its a commercially sold by ESP (pcbs), he didn't post it publicly.
regards
Prasi

To, xrk, please post it privately to spacehead.
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Old 15th March 2019, 09:24 PM   #112
xrk971 is online now xrk971  United States
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Quote:
Unfortunately it is not silent while source is connected and nothing is played.
There is some white noise with NE5532 and with TL072. It is audible. That is with 16 ohm sensitive earbuds, that I use to test for noise. With 32 ohm headphones it is better but not hifi level.
The source is quiet. Is this supposed to have pitch black noiseless background?
Sorry you are having some issues with noise. The way to check for noise independent of your source is to short the input connection to ground. Then listen - is it silent then? If not, and you hear hum or hiss, then the problem is the amp.

If you connect a source and turn the volume to zero and hear a little noise compared to the shorted input, the problem is your source. If you turn the volume up on your source and it is noisier, then the problem is definitely your source.

I measured a SNR of about 105dB, above 500Hz, below that, my PSU at the time was letting some mains hum in and SNR was about -80dB - typical for mains powered linear regulated PSU's.

What kind of PSU are you using? A non-optimized DC-DC SMPS migh have some hiss inherent in it unless you do a couple of things like add a CLC filter followed by a cap multiplier, followed by a CRCRC filter. Sort of like what we do in the Yarra preamp. You are also leaving off the output inuductor, although unlikely, this may be causing a mild oscillation that comes off as a hiss. Another source of mild osccilation are choice of compensation caps C5, i am using small 22pF NP0 SMT soldered to the TH pads. I see you are using some ceramic disc caps - are those NP0/C0G's rated at 22pF? If you have a scope, look to see if it oscillates and increas C5 to 27pF or 33pF to get the oscillation to go away.

Here is a photo of one of my ESP HPA's but fitted with Toshiba 2SA1837's and 2SC4793 instead of BD140/139's (hence pinouts are flipped). Running in Class AB, it did not really need a heatsink. I later boosted it to run Class A and fitted a sizable little heatsink to it. The PSU is a linear Nazar shunt reg from Prasi. I am using BB OPA2604AP and it works fine. I have built a second one with BD139/140's and it used NE5532's which also work fine.

ESP HPA-esp-nazar-toshiba-jpg
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Last edited by xrk971; 15th March 2019 at 09:29 PM.
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Old 15th March 2019, 11:07 PM   #113
Spacehead is offline Spacehead  Finland
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I have axial 1 uH inductors there. It seems it is quiet now. If I raise the OS volume the hiss starts, so it comes from the source apparently. Those 16 ohm earbuds , that are supposedly good quality are letting some environment noise from the computer fans through, that I most likely mistaken for noise coming from the earbuds. Luckily, the audio quality is very good, and noise is inaudible when listening something.


My PSU is LM7805/LM7905 adjustable to +15 - 15V. It comes from AC Wall wart, using this circuit:
http://musicfromouterspace.com/analo...WARTSUPPLY.php
That page design has 3300 uF x 6, I have 470 uF x 6. Half-wave rectification would most likely benefit from more capacitance.

The ceramic caps are 22 pF and NP0. The shop didn't have X7R or film capacitors at that value.



xrk971, you make clean work, it looks tidy!

Last edited by Spacehead; 15th March 2019 at 11:20 PM.
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Old 15th March 2019, 11:28 PM   #114
xrk971 is online now xrk971  United States
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Thanks! You are fine with 22p NP0 ceramic caps. Your PSU should be fine as well - it does sound like problem was your source. Wait until you get some good headphones. I like open back ones. They are going to be at least $100 for good ones.
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Old 16th March 2019, 01:29 AM   #115
asuslover is online now asuslover  Romania
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Are boards available for this amp?
Thanks
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Old 16th March 2019, 03:17 PM   #116
prasi is offline prasi  India
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Hello asuslover, xrk might have some spares . please PM him.
regards
Prasi
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Old 17th March 2019, 12:17 PM   #117
Spacehead is offline Spacehead  Finland
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Can you make the bias adjustable by replacing R6 (6.8K resistor) with a variable trimmer? If so, where you could then measure the bias current?
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Old 17th March 2019, 05:56 PM   #118
xrk971 is online now xrk971  United States
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R7 and R8 set the quiescent bias current. Just change to a lower value. Half lower is double the bias etc. Use a quality metal thin film resistor or carbon film but not woreqound or metal thick film. Trimmer is not good idea as there is heat here and it needs to be stable in value.
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Old 17th March 2019, 06:30 PM   #119
Spacehead is offline Spacehead  Finland
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Quote:
Originally Posted by xrk971 View Post
R7 and R8 set the quiescent bias current. Just change to a lower value. Half lower is double the bias etc. Use a quality metal thin film resistor or carbon film but not woreqound or metal thick film. Trimmer is not good idea as there is heat here and it needs to be stable in value.



I measured the voltage drop across R8 to be 25 mV. They are 10 ohm resistors. That means current that flows through is 2.5 mA.

But doesn't the voltage drop change if you change the resistor value too? Now what happens to the current flowing?


How did you calculate it?
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Old 17th March 2019, 11:15 PM   #120
xrk971 is online now xrk971  United States
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You should model it on LTSpice and play around with the DC setpoints. In general, the bias current is proportional to the emitter resistors. But this is a Class AB design so you won’t improve performance much by running higher quiescent current. If 2.5mA at 10ohm then 25mA at 1ohm and ~50mA at 0.47ohm, and 108mA at 0.22ohm. 108mA is all you need for staying Class A with most headphones. But you will need to use bigger TO-220 BJTs as 108mA at 15v is 1.62w per BJT.
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