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Using a line preamp to drive headphone
Using a line preamp to drive headphone
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Old 5th May 2015, 06:57 AM   #1
ginetto61 is offline ginetto61  Italy
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Default Using a line preamp to drive headphone

Hi !
i have an old Bryston line preamp not used.
I would like to modify it to drive a 600 ohm headphone by Akg.
I read in the specs sheet that the Z out of the preamp is around 50 ohm and i see that the output is AC coupled with a 4 uF film cap.
Only problem should be to change the output capacitor with a bigger one ?
Could i still use it as a line preamp after the mod ?

I found this very useful calculator

http://sim.okawa-denshi.jp/en/CRtool.php

Click the image to open in full size.

With C= 10 mF and R= 600 ohm (the headphone) i get a Cut-off frequency fc = 26 [Hz]

Now that i think a little i would like to leave the existing film cap and bypass it with a not-polarized 10-20 uF
Would this work ?
Thanks a lot for any advice, gino

Last edited by ginetto61; 5th May 2015 at 07:14 AM.
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Old 5th May 2015, 07:11 AM   #2
jcx is offline jcx  United States
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many recommend a 10x margin - so aim for 2 Hz rather than 20

at that size Al electrolytic is the most practical option - "bi-polar" version with considerably higher V rating than signal is the lower measured distortion option by Bateman's "Capacitor Sound" series
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Old 5th May 2015, 07:42 AM   #3
ginetto61 is offline ginetto61  Italy
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Quote:
Originally Posted by jcx View Post
many recommend a 10x margin - so aim for 2 Hz rather than 20
at that size Al electrolytic is the most practical option - "bi-polar" version with considerably higher V rating than signal is the lower measured distortion option by Bateman's "Capacitor Sound" series
Hi thanks a lot indeed for the very kind and valuable reply
So the idea is not that crazy ?
I have thought about this after seeing that many solid state preamps have impressive measurements (for what this can count ) also when asked to drive a heavy 600 ohm load
I understand that a real world HP is not a pure resistive load but the Bryston seems to have a robust output stage.
It is a discrete op-amp design, a very classical choice for Bryston.
Thanks a lot again, gino

Last edited by ginetto61; 5th May 2015 at 07:48 AM.
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Old 5th May 2015, 08:19 AM   #4
AndrewT is offline AndrewT  Scotland
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150uF into 600ohm gets you into the right passband territory.

You could use a 150uF bi-polar alone, or two 330uF bi-polar in back to back series for an effective 165uF, or two 330uF polar in back to back series connected, or four 150uF polar in series/parallel connection with each half of the parallel inverted, one half +to+ and the other half-to-.

25V electros will be high enough rating for a +-15Vdc supplied headphone amplifier.
If you use >20Vdc single supply polarity, then use 35V or 50V electros.
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Last edited by AndrewT; 5th May 2015 at 08:21 AM.
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Old 5th May 2015, 08:38 AM   #5
ginetto61 is offline ginetto61  Italy
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Quote:
Originally Posted by AndrewT View Post
150uF into 600ohm gets you into the right passband territory. You could use a 150uF bi-polar alone,
Hi and thanks a lot indeed.
I like this solution much more ... any specific suggestion of Brands/series ?

Quote:
or two 330uF bi-polar in back to back series for an effective 165uF, or two 330uF polar in back to back series connected, or four 150uF polar in series/parallel connection with each half of the parallel inverted, one half +to+ and the other half-to-.
25V electros will be high enough rating for a +-15Vdc supplied headphone amplifier.
If you use >20Vdc single supply polarity, then use 35V or 50V electros.
Voltage rails are +/-24V stabilized with fixed voltage regulators.
I would like to solder them on the existing ones (4uF PP caps) because the traces on the pcb are extremely delicate. I replaced the original MKC 3.3 uF some years ago risking to damage the traces ...
I really would like to be in the brain of some designer when they decide the size of the traces. I like bigger and more robust copper lines.
I have already noticed that the quality on some DIY pcbs are so much better than those on even not cheap commercial units.
This is sincerely unbelievable ...
Thanks a lot again, gino

Last edited by ginetto61; 5th May 2015 at 08:43 AM.
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Old 5th May 2015, 09:44 AM   #6
ginetto61 is offline ginetto61  Italy
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However, this is a much newer Bryston preamp
Click the image to open in full size.
i see some blue decoupling caps that indeed look like electrolytic.
But i do not have the schematic to check.
This one is balanced while mine is unbalanced, but there is always gain stages made with discrete op-amps.
Thanks again, gino
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Old 5th May 2015, 11:05 AM   #7
abraxalito is offline abraxalito  United Kingdom
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Quote:
Originally Posted by ginetto61 View Post
Voltage rails are +/-24V stabilized with fixed voltage regulators.
You mean to say they're using 3pin IC regulators? If so then with your 600ohm load there will likely be significant noise generated on the supply lines as IC regs have high-ish output impedance at relatively low currents. I suggest adding more decoupling caps if you like your audio served with its original dynamics
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Old 5th May 2015, 12:04 PM   #8
ginetto61 is offline ginetto61  Italy
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Quote:
Originally Posted by abraxalito View Post
You mean to say they're using 3pin IC regulators?
Hi thanks for the reply and yes. 3pin IC regulators ... fixed voltage.

Quote:
If so then with your 600ohm load there will likely be significant noise generated on the supply lines as IC regs have high-ish output impedance at relatively low currents. I suggest adding more decoupling caps if you like your audio served with its original dynamics
do you mean caps close to the circuit ? how many uF per rail per channel ?
more or less.
Thanks a lot, gino
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Old 5th May 2015, 12:39 PM   #9
abraxalito is offline abraxalito  United Kingdom
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I mean caps across the supplies to the discrete opamp you mentioned. Closest to the output devices. I'd suggest determining how much capacitance by ear - add until you're satisfied with how it sounds. Start off with 2,200uF per rail per channel.
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Old 5th May 2015, 12:47 PM   #10
nattawa is online now nattawa  Canada
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AKG's 600-ohm cans have low sensitivity and require high level driving capacity to sound right. You may want to make sure your preamp is capable of at least 10Vp-p output into 600-ohm loads.

I have K240DF, also 600-ohm units, and I drive them with "The Wire SE-SE" designed by Owen. The original The-Wire is a high performance, powerful line buffer with unity gain and was not able to make a noise at normal DAC output level, until I modified the gain setting to give it a +12dB gain to make the phones really sing and slam.
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