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Old 14th November 2008, 02:12 PM   #21
semola is offline semola  Italy
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Hallo Mr. TimS,

Some questions for You...

- I don't understand if Your last version of the buffer is a pratical realization or is on the paper( or simulator) Yet

- Is possible to use up to +/- 22V supply voltage and if yes,this need some modifications?

-What transistors should be matched and what should be in thermal contact?

-I think this improved buffer should be perfect in combination with a high performance opamp like LME49870 in Yung multiloop(or traditonal)configuration to achive an impressive headphone amplifier for low impedance type.
If this idea can be intriguing for You and the community,if You like,can You simulate the circuit and share the results maybe in a new thread?

Thank You very much in advance and best compliments for Your
very interesting circuit.

Semola
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Old 15th November 2008, 08:13 PM   #22
TimS is offline TimS  New Zealand
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Hi Semola

Quote:
Originally posted by semola

- I don't understand if Your last version of the buffer is a pratical realization or is on the paper( or simulator) Yet
The circuit has just been simulated. I fully intend to build it but i have been waiting for parts, which will arrive next week.
Quote:

- Is possible to use up to +/- 22V supply voltage and if yes,this need some modifications?
The circuit will work fine at 22V, the bias current will increase slightly to fix this increase R3 from 4k to 6k.
Quote:

-What transistors should be matched and what should be in thermal contact?
Ideally Q4, Q19, Q3, Q20 should be matched. The current through Q1 and Q2 will increase with temperature It could be compensated for by putting Q4 in thermal contact with Q2 and Q3 with Q1, but this is not ideal since it will ruin the matching between the Q4 and Q19 etc. A better option would be to add thermistors across R4 and R5 which will turn the bias down if Q1 and Q2 get too hot.

Quote:

-I think this improved buffer should be perfect in combination with a high performance opamp like LME49870 in Yung multiloop(or traditonal)configuration to achive an impressive headphone amplifier for low impedance type.
If this idea can be intriguing for You and the community,if You like,can You simulate the circuit and share the results maybe in a new thread?
This is originally what I designed this circuit for, a buffer that I can add to a high performance opamp so it can drive some headphones. But I found that this circuit has superb performance all by itself.
Some of its specs:
3dB Bandwidth = 70MHz
Gain = 0.997
Input Impedance = 6M
Output Impedance = 0.065R

If R4 and R5 are 300R the bias is set to a high 220mA
total dissipation = 6.4W but
Distortion = 0.001% for +- 13V @ 1kHz into 500R
Distortion = 0.001% for +- 13V @ 1kHz into 100R
Distortion = 0.006% for +- 13V @ 1kHz into 32R
Distortion = 0.004% for +- 13V @ 10kHz into 100R
Distortion = 0.007% for +- 13V @ 10kHz into 32R
Distortion = 0.1% for +- 12V @ 1kHz into 8R

But if R4 and R5 are 150R then the bias is a lower 48mA
total dissipation = 1.4W
Distortion = 0.001% for +- 13V @ 1kHz into 500R
Distortion = 0.01% for +- 13V @ 1kHz into 100R
Distortion = 0.045% for +- 13V @ 1kHz into 32R
Distortion = 0.004% for +- 13V @ 10kHz into 500R
Distortion = 0.01% for +- 13V @ 10kHz into 100R
Distortion = 0.045% for +- 13V @ 10kHz into 32R

I have done some simulation with the buffer inside a lesser opamp's feedback loop but with worse performance, but Microcap 6 is fairly old now and doesn't include the new high performance opamps.
I checked the datasheet for the LME49870 and this buffer would be a perfect match, the bandwidth is about the same and the opamp noise and distortion is very good.

Quote:

Thank You very much in advance and best compliments for Your
very interesting circuit.

Semola
Thanks for your interest

Cheers

Tim
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Old 17th November 2008, 12:51 AM   #23
lineup is offline lineup  Sweden
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Hi Tim.

Here you can see some experimental evaluation
that I have done with Diamond follower.

- Single supply, input and output capacitor
- Resistors + Bootstrap instead of current source
- 100% Class A operation. 20 mA idle
- High impedance input ~500kOhm

* My new idea is the U2, U6 cascoding transistors.
This actually improves. But not as much as I thought.

* Bootstrap C2, C5 improves much more.
And R1, R15 should be considerably larger in value
than R2, R14 for the optimal Bootstrapping point.

* The cascode in output U1, U8 does not seem to improve things much.
Not what I have found out so far.

* C1, C6 are good for the high frequency behavior.

The performance is very good.
Even at high output voltage swing.
Say at +-20 Vrms = +- 30 Vpeak!!!!!


Lineup regards
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Old 17th November 2008, 09:53 AM   #24
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Nice circuit lineup.

A slightly different buffer but which can also give excellent results.
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File Type: jpg buffer.jpg (28.3 KB, 1764 views)
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Old 17th November 2008, 10:07 AM   #25
lineup is offline lineup  Sweden
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I would think it is very good your buffer, yes.

The great thing is it should be much, much easier
to make zero Offset, for DCin-DCout operation.
As your circuit matches NPN with NPN and PNP with PNP.

The standard Diamond has to match NPN Vbe with PNP Vbe in output.
Which can be very tricky to adjust when using DCin-DCout.
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Old 17th November 2008, 11:51 AM   #26
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Its a very useful feature to have especially with headphones where one would ideally want 0 offset. It displays comparable specs to similar complexity diamond buffer. With the values shown it should be in Tim s ballpark figures. For heavier loads the diamond can be tweaked like your circuit to give exellent results. I use it to buffer the signal from car radio to amps because of the long interconnects to amps being installed in the boot and because it can drive several amps in parallel. Not really my circuit idea, saw a simplified version on the net, cant recall where and I just added the mirrors and cfp s to enhance performance.
Elegant circuits that display virtually no sound of their own, what comes in, is what comes out.
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Old 17th November 2008, 03:09 PM   #27
jam is offline jam  United States
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Have you guys tried to use complementary jfets for the input of the buffer, Helps offset drift as you don't have to deal with base currents and could be direct coupled.

Jam
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Old 17th November 2008, 04:21 PM   #28
juma is offline juma  Germany
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Hi Jam, it's nice looking (and even better working) circuit. Just one thing - check the J74's polarity, drain should be connected to -12V rail, although JFETs are known for their source/drain interchangeability.
I run this buffer with 5mA through JFETs (7.2 mA Idss, matched) and 7 mA through output BJTs (BC550c/BC560c) - that's enough for 1K load.
DC offset is easily adjusted with R1 or R2 and once adjusted it stays stable.
Also, I used simple CCS - two 1N4148 diodes instead of LED - it sounds great.

Best regards !
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Old 17th November 2008, 04:41 PM   #29
jam is offline jam  United States
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Hi juma,

Thanks for the correction....I hd pulled from the web. It is a great sounding buffer.

I have cascoded the current sources with some audiable improvement, just add another led (or two diodes) and transistor in series with the corrent source.

Best regards,

Jam
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Old 18th November 2008, 06:55 AM   #30
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Quote:
Originally posted by jam
Have you guys tried to use complementary jfets for the input of the buffer, Helps offset drift as you don't have to deal with base currents and could be direct coupled.

Jam
Yes, I've tried this using SK170/SJ74. Both as output buffer, and as input buffer in my le classe A headphone amp. I know most of you don't believe this, but JFETs sound different from bipolars. It's a matter of taste.
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