Minimalist balanced phono preamp?

[leadbelly]

I did quite a lot of searching through prono preamp schematics today, both here and on Google, hoping to find what I was looking for: an SS minimalist balanced MM phono preamp. Either discrete or opamp would have been fine, I want it to be balanced and low parts count. Believe it or not, even with the dozens of designs out there, I could not find a single example of what I was looking for. Plenty of balanced discrete designs, but BIG parts counts. Plenty of opamp designs, but all single ended. I'm just hoping someone here knows of one that missed my long search.

[real]

Hi,

I have just recently built a preamp with a balanced phono section. It has a FET differential input stage, with CCS, functioning as simply a gain stage. The FETs are low noise (2SK389), which combined with the stages gain gives good noise performance of the preamp. The output is then converted to single ended (taking the output from only one of the drain resistors - a proper balanced to SE convertor could be used, but the way I have it works fine for me). An emitter-follower stage is also used here to give low output impedance to allow the gain in the next stage to be set easily.

This single ended signal is then fed into a standard active opamp filtering stage, which I found at http://www.beigebag.com/case_riaa_3.htm (using only the second opamp section). That's pretty much it.

Please note also that the input FETs require about +75VDC rail, in order to be able to use a high enough drain resistor to get high enough gain (without using a very low current which would sacrifice noise performance). To get this, as well as the standard +/- 15V rails, I used a voltage quadrupler on my transformers 15-0-15 VAC output to get ~90VDC which is then regulated to 75VDC. You could use a separate transformer, but this may not be convenient.

If required I can post schematics for you.

Regards,

Chris.

[planet10]

Originally posted by real
Please note also that the input FETs require about +75VDC rail

In Canada at least, the old fashioned razor plugs in bathrooms had a suitable 120V-120V isolation trafo... i find them fairly regularily at thrift stores & garage sales for $1-2 (and if i get desperate know where there are a lot for $5 each)... you'd need a reg after it, but i assume you'd want one anyway.

dave

[AndrewT]

Hi,
Ben Duncan did at least 2 series on hi-end preamps (Wireless World, new title now maybe electronics & W.W.) with balanced phono stage, mainly for MC. They were written 80's (maybe early 90's) when the chosen opamps were very expensive. Mind you the best are still expensive. But it was all designed as plug-in modules so you can just use the one pair of modules.
As I recall very high component count.

Chris, please post your schematic.

[leadbelly]

I'm not sure I understand the benefit of having a differential gain stage only and then immediately converting to single ended in the proceeding stage. Does the differential gain stage alone improve common mode noise rejection? With or without a transformer?

[planet10]

Originally posted by leadbelly
I'm not sure I understand the benefit of having a differential gain stage only and then immediately converting to single ended in the proceeding stage. Does the differential gain stage alone improve common mode noise rejection? With or without a transformer?

Given that your line stage is differential, as is your planeed power stage, it makes no sense to collapse a balanced phono stage to SE.

dave

[leadbelly]

Originally posted by planet10
In Canada at least, the old fashioned razor plugs in bathrooms had a suitable 120V-120V isolation trafo... i find them fairly regularily at thrift stores & garage sales for $1-2 (and if i get desperate know where there are a lot for $5 each)... you'd need a reg after it, but i assume you'd want one anyway.

dave

Stop kicking me in the @#$%^& with that tip! I pulled all the razor plugs in my family's houses out to put in GFCI's years ago, and it didn't occur to me to save a single one! I've yet to come across one cheap since.

[planet10]

Originally posted by leadbelly
Stop kicking me in the @#$%^& with that tip!

:^)

[real]

Hi,

As requested, I have attached the schematic. To avoid any possible copyright infringements I did not include the active equalisation section - it is just as in the link in my previous post. Note that the zener diodes provide protection against exceeding the 50V drain-source voltage of the FETs and also against the high input transient to the opamp at turn on.

To leadbelly: the purpose of having a differential input to me is to improve the CMRR and also to prevent induced hum on the line from reaching the output. Because of the amplification the FETs provide, any further induced hum or noise will be much smaller in comparison to the signal, hence a high SNR. I converted to SE here because I could not find out how to actively equalise in a balanced fashion - very little information is available on this (after much googling). I don't really like passive eq that much - I don't think it has as good noise performance (someone please correct me if I'm wrong) - so I switched to SE for the equalisation (the system that the preamp is for is SE anyway, so I would have had to convert sooner or later).

If anyone has any further questions I would be happy to answer them.

Regards,

Chris.

[leadbelly]

Originally posted by planet10


:^)

Sorry, man, didn't meaning anything bad, just joshing.