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Old 9th March 2010, 11:12 PM   #1011
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I am still working on a BJT version of the Discreet INA, theoretical that is because my fried board is not working still. I got the input stages working but not the rest. I substituted a lot of elcaps but that board is crowded so i started to build a new one from scratch. The circuit shown should allow DC coupling of the input. Bias is taken from output of OP1, OP2. The OPA134 servo allows finer adjustment with a 100kOhm trimmer.
I am also building a high quality headphone amp to listen to pasive components and make listening posible late at night when the family is sleeping.
i started to work on an rbb measurement setup and i am planning a RIAA encoder.
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Old 9th March 2010, 11:13 PM   #1012
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Here is the DC coupled BJT INA.
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Old 10th March 2010, 10:37 AM   #1013
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Location: Germany
About relais ( I saw you were talking about): a lot of good relais are no more available these days, a pity. This has to do with RoHs and WEEE here in Europe. But since most younger people nowadays listening to mp3 sounds only, it may not longer be necessary to deliver good sounding parts. Btw. Fraunhofer Institute (mp3 inventor) says that's nothing wrong with mp3 sound qualitity. I did not knew that my ears are so bad, because of I do not like the wimpy, bloodless mp3 sound ;-)

From the current available relais types a nice sounding one is the Panasonic ASX (e.g. the ASX20012). Min. switching capacity is as low as 10µA and 1mV (instead of the standard 10mV). Additional I found out that relais with particularly low thermoelectric voltage and thermal electromotive force sounded better to my ears. Pickering switches are making such devices as well, but they do not last very long ;-)
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Old 10th March 2010, 08:48 PM   #1014
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I started to build a headphone amp to make listening late at night posible and also to listen to passive components. Because i am also a hobby musician i own a lot of different phones from AKG and Sennheiser.
I have the HD280pro, AKG501 modified and AKG701 for example.
My main phone is the AKG701. It has quite a low impedance so I will use a diamond buffer
with ca.30mA class A as output stage. It can swing 50mW class A into the 60 Ohm AKG.
The buffer is a more or less clasical design with improvements to the current source that are similar to the High Z MPP i posted lately. A Fet constant current source feeds a green Led. The output is lowpass filtered by a cap and resistor to the bias transitor.
That arangement is very stable and after trimming offset with P1 it stays under 0.1mV.
As input buffer i use the PSG buffer with offset trim and i am still working on the voltage amplifying stage. I am trying to build a kind of I/U converter with current mirrors so the whole amp is global feedback free.
Attached Files
File Type: pdf JG Headphone Diamond Buffer.TSC - TINA.pdf (42.3 KB, 109 views)
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Old 10th March 2010, 10:03 PM   #1015
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Default New Headphone thread.

Hi Joachim,


I think it's better to start a new thread with your Headphone amp.
It would be a pity to talk about this headphone amp in your own MPP thread which is a tremendous phono amp.

BTW this is a similar amp but only with a different front (read: no led's) and output 2x.

Best regards,

Audiofanatic
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File Type: jpg dDB_isb_v022_schem.jpg (148.7 KB, 411 views)
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Old 10th March 2010, 11:02 PM   #1016
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I have seen that topology. I found it when i googled "Diamond Buffer". It should work as well. The cap-resistor lowpass i am using should work with that too. You loose a little bias voltage but i use BC550C/BC560C here that have current gain over 600 in my samples. I really think this little project is too small for it“s own thread and i already opened another worm of cans : "MPL", My Privat Speaker.
Anyway, i decided on a different topology. An OP amp is now doing the work in shunt mode. This way i can also listen to OP amps but i fear that even the NE5534 is good enough because in shunt feedback i only use the basic transfer linearity that is very good in most. Some with FET input have trouble there but i expect no enlightenment.
One channel of the Headamp is already working and noise is not audible.
I will build the other channel and then report.
Here comes the schematic but i have no plans to optimise it much any further.
Of cause i can play with iddle current and thinks like that but i think the way i made it is just fine for my pupose. If questions arise i will of cause answer.
P.S. : the amp is phase inverting so i may cut the leads of one of my phones and seperate the grounds.
P.P.S. : i use a 10kOhm pot on the input to get good speed with the Fet. I nulled the offset of the output stage first with the 1kOhm trimmer and nulled the compleet amp with the 100 Ohm trimmer. Offset is very stable and under 1mV.
Attached Files
File Type: pdf JG Headphone Amp.TSC - TINA.pdf (61.5 KB, 91 views)
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Old 11th March 2010, 09:00 PM   #1017
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i plan to build an MPP for a cartridge with very low output so here come the Ultra Low Noise High Z Fet MPP.
Some resistor values are halved. Trimmimg works the same as already posted.
Attached Files
File Type: pdf High Z MPP Fet Ultra low Noise.TSC - TINA.pdf (77.8 KB, 104 views)
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Old 12th March 2010, 03:40 AM   #1018
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The Headphone Amp is up and running.
It is dead quiet and PSU rejection is very good. I ran it on 8.5V negative and 12.5V positive without trouble.
The sound is extremely analytic so flaws in the program material stand out quite uncomfortable. I am surprised how much reverb is used on most productions. Analog recordings show frequency modulation and noise in high measure. Digital recordings with old A-D converters are not unproblematic too because a lot of granularity is audible. All in all this type of listening gives me new information.
When the recording is good the result is intense.
If i whould do a commercial version i whould set the amp on a servo and substitude the Fet buffer with an Opamp. The Fet current source could simply replaced by a 10 to 20kOhm resistor without much compromise. That way no adjustment is necessary.
If the source impedance whould be known, a compensation resistor could be added in the buffer.
Attached Files
File Type: pdf JG Headphone Amp com.TSC - TINA.pdf (57.7 KB, 82 views)
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Old 13th March 2010, 12:16 AM   #1019
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I got a decent Digital Camera so tomorrow i will post some picturs of what i build recently.
I get a scope and function generator for my homelab so i will be able to post some measurements without the need of my professional lab.
That stuff has got so cheep that even i can afford it.
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Old 13th March 2010, 12:21 AM   #1020
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Joachim,
It's 2:20, go to bed!

Regards, Allen
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