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Old 21st April 2007, 06:08 AM   #1
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Default Discrete Op Amp Ideas for a Preamp

Many, many years ago I built a JC-2 clone based on the schematics in the "Audio Amateur". Since then I have learned a few things, including how to run HSPICE. For those of you who have not yet learned this tool, I highly recommend it if you ever plan on doing any design.

I am desiging a preamp that is conceptually a follow-on the the JC2, in that it will use discrete op amps. Since I have a large LP collection, it will include a MC pre-preamp stage as well as a line amp stage.

Here is a brief preview of the design.

* Reed relays for all signal switching
* Satellite power supply to minimize magnetic interference

* MC-preamp with En less than that of a 100 ohm resistor
* MC preamp O/L BW flat to >100 KHz, C/L BW flat to >1 MHz
* Teflon caps in signal path , including RIAA network
* 1% RN55 metal film resistors throughout
* MC preamp distortion < noise floor up to onset of clipping

* Line amp capable of driving 100 ohm load
* Line amp distortion below noise floor
* Ability to drive up to +/- 20V peak

I have already simulated the MC preamp, and with the use of low noise matched PNP/NPN devices, there appears to be no problem meeting the noise floor. Additional En reduction may be possible by paralleling input devices (En is reduced by RSS sum).

The next step is the line amp. Since the line amp's source impedances (volume control, for example) are in the 5-10Kohm range, it is right on the cusp between BJTs and JFETs in terms of noise floor at the input stage. I will need to evaluate both types of devices to determine the best choice.

Stay tuned...

JCM
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Old 21st April 2007, 06:20 AM   #2
Ricky is offline Ricky  United Kingdom
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Brilliant!!

I like your ideas, especially the relays. I hae a design soon to go on my website. 4 inputs, tape in and out, Burr Brown output stages ( balnced and un-balanced). Reed relays used throughout including tape monitor.

Looking forward to your progress.


Ricky.
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Old 21st April 2007, 06:51 AM   #3
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Do you have some practical results except simulation ? Why " satelite " caps ? - final sound much more depend on premeditaion of connection... BTW, 100 Ohm driving capability is lowly demand - try to think about ten Ohms..
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Old 21st April 2007, 07:17 AM   #4
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Upupa,

The ability to drive 100 ohms should be more than sufficient. A typical preamplifier load consists of the resistive input impedance of the amplifier (typically 20 K ohms) plus the paralled capacitance of the amplifier's input devices and the capacitance of the cable,. Most small signal transistors have Cb < 100 pf. If we assume a common coax such as RG58U, it has a capacitance of 100 pf/meter. The transistors, therefore contribute a capacitance equal to approx 1 meter of additional coax. So, even if we were to drive 10 meters of cable (a lot for a home stereo system), the capacitive reactance Xc would be approx 7K ohms for a 20 KHz signal. Xc = 1/(2*pi*C*f). Add to that the paralleled amp DC resistance, and you get a load of approx 5K ohm. If we repeat this procedure for 20 Hz, we get an load impedance of approx 20K ohms.

There is an insertion loss difference between 20 Hz and 20 KHz caused by the variation in Xc with frequency, but it is very small. You can calculate it by considering the ratio of voltage dividers at 20 Hz and 20 KHz. (100 ohms driving 5K vs. driving 20K). If we use a DC Rl of 20K, then the insertion loss difference between 20 Hz and 20 KHz is 0.13 dB, which is below audability, at least for me.
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Old 21st April 2007, 07:23 AM   #5
Zoran is offline Zoran  Serbia
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Try bistabile relays digitaly triggered, for all switching
siemens had one model with all gold contacts,
I am not shure but i think that Zetler now have the same model,
but different marking...
cheers
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Old 21st April 2007, 08:00 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Upupa Epops
Do you have some practical results except simulation ? Why " satelite " caps ? - final sound much more depend on premeditaion of connection... BTW, 100 Ohm driving capability is lowly demand - try to think about ten Ohms..
Hi,
I too think that 100ohm driving capability is adequate for a pre-amp.
It will allow direct drive of 600ohm inputs as well as all those higher than that.
It implies a source impedance of under 10ohms, again exemplary.
The output power requirement (4Wpk) will need medium power output transistors in the range 6W to 10W, what do you have in mind?

Upupa,
can you explain why we should need even more current ability out of a pre-amp? (+-20Vpk into 10r = a 20W ClassA power amplifier)
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Old 21st April 2007, 09:29 AM   #7
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By our experiences ( PMA, me and most of Czech DIY comunity ) is very low output impedance ( less than ten Ohms ) mandatory for really good listening... BUF 634 ( or similar ones ) or more precisely discrete one ( like WJ's diamond buffer ), connected into feedback loop of opamp give best results. Also TPA 6120A is excellent....Or you can make it all discrete, but very low output impedance is mandatory, belive or not...
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Old 21st April 2007, 10:05 AM   #8
PMA is offline PMA  Europe
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I agree with Upupa, that low distortion link output stage with high current drive capability (like 200mA - 500mA) gives the best listenning results. With output impedance defined by series resistor 5 ohm - 10 ohm.
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Old 21st April 2007, 04:09 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
+-20Vpk into 100r is 200mApk.
where's the problem?


Are you confusing load impedance with output impedance?
Quote:
With output impedance defined by series resistor 5 ohm - 10 ohm.
this is clearly output impedance and with this Rs one can drive >=100r if the output stage can pass sufficient current.

But Upupa said
Quote:
100 Ohm driving capability is lowly demand - try to think about ten Ohms..
which to me means load impedance and in reply followed up with
Quote:
is very low output impedance ( less than ten Ohms ) mandatory...........but very low output impedance is mandatory
where Upupa is referring to Rs (output impedance).

So I say again.
A stage designed to drive 100ohms to +-20Vpk should do an excellent job of driving all inputs of >=600ohm.

Give me a reason why the output stage should be designed to drive +-20Vpk into 10ohms.
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Old 21st April 2007, 04:17 PM   #10
PMA is offline PMA  Europe
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Quote:
Originally posted by AndrewT


Are you confusing load impedance with output impedance?
No, I am not. There is no problem, in case you get 200mA output current with minimal distortion.
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