DIY phono preamp - cheap and simple! - Page 31

Markje · 24th July 2007, 05:33 PM

All the stuff is ordered, I'm hoping to get it at the end of the week. I already have the power supply, from another project (which didn't work

). It's a +/-15V supply with zener diodes (I heard from someone it is a good regulator, better than 78xx and 79xx; we'll see). As soon as it is finished, I'll post some pictures.

Regards,

Mark

rjm · 24th July 2007, 07:35 PM

Mads,

DC gain is indeed 1000. I glad at least someone was on the ball today.

Fran,

In principle you can use a step-up transformer with the VSPS. I have done so myself. Normally it is installed in front of R1, leaving R1 in place, but replacing the normal value of 47k with something that gives the correct cartridge loading (for the MC cart being used) when reflected back across the transformer.

There is not normally any resistor on the input side of the transformer.

The value of R1 is found by multiplying the recommended cartridge load by the square of the turns ratio (i.e. voltage gain^2).

Ex. 100 ohm recommended load, 20 dB (10x) voltage gain [1;10 turns ratio] of the transformer, R1=100x10^2=10k.

That your transformer has a switch for 30 or 100 ohms I presume means that in the box containing the transformer there is also already resistors across the output winding selected to give the stated reflected input impedance. If thats the case you dont need R1 in the VSPS, since this is duplication, but if the resistors with the transformer are 10k or less there is no harm done leaving R1 in the VSPS at 47k, since connected in parallel R1 doesn't significantly affect the total load.

So to recap: normally R1 is set to provide the correct input impedance for the cart. If this already done elsewhere, you can either leave R1 as it is or remove it ... dont "short it" with wires!

Finally, the reason the VSPS may not be ideal with an input transformer is that the input impedance of the cartridge as seen by the op-amps inputs is (turns ratio)^2 larger than the actual cartrdge internal impedance. Say you used a 40 ohm cart, 1:10 voltage/turns ratio, you are essentially using a 4k cartridge... this will induce some extra current noise which will actually increase the output noise measurably when a bipolar op-amp is used.

So to recap that part: generally speaking the HiZ version of he VSPS is preferred for this application.

And I feel duty bound to add that Phonoclone boards that are drop-in replacement boards for the VSPS boards are available, or the layout is at any rate. As an upgrade path its pretty economical, as the power supply and hardware is unchanged.

Markje,

Ok, good luck and I am looking forward to seeing the photos!

/R

woodturner-fran · 25th July 2007, 10:03 AM

......... fran scuttles off to have a look at the phonoclone.......

Thanks richard

rjm · 26th July 2007, 01:09 AM

The latest Phonoclone layouts are posted to the Help Desk. I'm working on a web page update to bring the products section in line with the version changes, but its not finished yet.

Markje · 26th July 2007, 06:21 PM

Today the mailman came with a package containing everything to build the VSPS.

After about a few hours of working it was finished. I already had the power supply, so I only had to build the amp itself. I was only able to test it at the computer with crappy speakers. Tomorrow I have a day off, so I will test it at my set, but I'm confident it will sound just fine.

Pictures are below:
This is the actual amp, resistors are 1W, since it was cheaper than 0,5W and 0,25W resistors:

This is the power supply, below the C are the zeners:

Both cases are connected:

Rear view:

Front view:

rjm: thanks for the schematic, I don't think it can get any easier... So, anyone who is looking for a simple phonoamp; try this, easy to build and it sounds good.

Regards,

Mark

woodturner-fran · 26th July 2007, 07:37 PM

RJM, do you have any phonoclone boards? I just had a thought that I could stack them on top of the VSPS ones and add an extra set of inputs and outputs...

Don't even have an LOMC cart yet!!!

Fran

rjm · 27th July 2007, 12:31 AM

Don't stack them.. trust me that's begging for trouble.

Either build a new case for the phonoclone or replace the VSPS boards in the old case.

I don't have any boards at the moment, sorry...

ok, actually I do, my Phonoclone B.E. ... since I've been using the mini RC version this is my backup and I only use it for comparison now ... its prebuilt (duh) but I can sell you that if you like, email me about it if you are interested. You provide the power supply, of course...

arnoldc · 27th July 2007, 08:07 AM

Quote:

Originally posted by woodturner-fran
Another question for RJM/ VSPS users:

I'm thinking of going the MC route and wanted to continue to use my VSPS. However gain will be an issue - so I'm planning on using a step up Tx with the VSPS. The particular one I have in mind has a switch for loading at 30 or 100 ohms for the MC cart. Can I just short out R1 with some wire so that the only loading is at the step up?

Also, has anyone else done this and how did it sound for you?

Fran

Hi,

I have tested the VSPS with a step up. It was a mega-buck Conrad Johnson step-up. I did not do anything but to connect the CJ ground to the VSPS ground. It sounds very good.

I did not bother about the 47K at the input... Don't know if I should have, but it was just a test.

rjm · 27th July 2007, 10:19 AM

For a number of reasons its a good idea to leave R1 as 47 kohms, as a general rule that's pretty much what the designers of step up transformers expect to be loading the output.

rjm · 28th July 2007, 03:42 AM

fran,

I just found the two unused Phonoclone mini (LM317) boards in my desk drawer! Contact me if interested.

28th July 2007, 03:42 AM	#310
rjm Richard Murdey diyAudio Member Join Date: May 2004 Location: Kyoto Blog Entries: 72	Phonoclone mini fran, I just found the two unused Phonoclone mini (LM317) boards in my desk drawer! Contact me if interested.

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24th July 2007, 05:33 PM	#301
Markje diyAudio Member Join Date: Jan 2004 Location: The Netherlands	All the stuff is ordered, I'm hoping to get it at the end of the week. I already have the power supply, from another project (which didn't work ). It's a +/-15V supply with zener diodes (I heard from someone it is a good regulator, better than 78xx and 79xx; we'll see). As soon as it is finished, I'll post some pictures. Regards, Mark

24th July 2007, 07:35 PM	#302
rjm Richard Murdey diyAudio Member Join Date: May 2004 Location: Kyoto Blog Entries: 72	Mads, DC gain is indeed 1000. I glad at least someone was on the ball today. Fran, In principle you can use a step-up transformer with the VSPS. I have done so myself. Normally it is installed in front of R1, leaving R1 in place, but replacing the normal value of 47k with something that gives the correct cartridge loading (for the MC cart being used) when reflected back across the transformer. There is not normally any resistor on the input side of the transformer. The value of R1 is found by multiplying the recommended cartridge load by the square of the turns ratio (i.e. voltage gain^2). Ex. 100 ohm recommended load, 20 dB (10x) voltage gain [1;10 turns ratio] of the transformer, R1=100x10^2=10k. That your transformer has a switch for 30 or 100 ohms I presume means that in the box containing the transformer there is also already resistors across the output winding selected to give the stated reflected input impedance. If thats the case you dont need R1 in the VSPS, since this is duplication, but if the resistors with the transformer are 10k or less there is no harm done leaving R1 in the VSPS at 47k, since connected in parallel R1 doesn't significantly affect the total load. So to recap: normally R1 is set to provide the correct input impedance for the cart. If this already done elsewhere, you can either leave R1 as it is or remove it ... dont "short it" with wires! Finally, the reason the VSPS may not be ideal with an input transformer is that the input impedance of the cartridge as seen by the op-amps inputs is (turns ratio)^2 larger than the actual cartrdge internal impedance. Say you used a 40 ohm cart, 1:10 voltage/turns ratio, you are essentially using a 4k cartridge... this will induce some extra current noise which will actually increase the output noise measurably when a bipolar op-amp is used. So to recap that part: generally speaking the HiZ version of he VSPS is preferred for this application. And I feel duty bound to add that Phonoclone boards that are drop-in replacement boards for the VSPS boards are available, or the layout is at any rate. As an upgrade path its pretty economical, as the power supply and hardware is unchanged. Markje, Ok, good luck and I am looking forward to seeing the photos! /R

25th July 2007, 10:03 AM	#303
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	......... fran scuttles off to have a look at the phonoclone....... Thanks richard

26th July 2007, 01:09 AM	#304
rjm Richard Murdey diyAudio Member Join Date: May 2004 Location: Kyoto Blog Entries: 72	The latest Phonoclone layouts are posted to the Help Desk. I'm working on a web page update to bring the products section in line with the version changes, but its not finished yet.

26th July 2007, 06:21 PM	#305
Markje diyAudio Member Join Date: Jan 2004 Location: The Netherlands	Today the mailman came with a package containing everything to build the VSPS. After about a few hours of working it was finished. I already had the power supply, so I only had to build the amp itself. I was only able to test it at the computer with crappy speakers. Tomorrow I have a day off, so I will test it at my set, but I'm confident it will sound just fine. Pictures are below: This is the actual amp, resistors are 1W, since it was cheaper than 0,5W and 0,25W resistors: This is the power supply, below the C are the zeners: Both cases are connected: Rear view: Front view: rjm: thanks for the schematic, I don't think it can get any easier... So, anyone who is looking for a simple phonoamp; try this, easy to build and it sounds good. Regards, Mark

26th July 2007, 07:37 PM	#306
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	RJM, do you have any phonoclone boards? I just had a thought that I could stack them on top of the VSPS ones and add an extra set of inputs and outputs... Don't even have an LOMC cart yet!!! Fran

27th July 2007, 12:31 AM	#307
rjm Richard Murdey diyAudio Member Join Date: May 2004 Location: Kyoto Blog Entries: 72	Don't stack them.. trust me that's begging for trouble. Either build a new case for the phonoclone or replace the VSPS boards in the old case. I don't have any boards at the moment, sorry... ok, actually I do, my Phonoclone B.E. ... since I've been using the mini RC version this is my backup and I only use it for comparison now ... its prebuilt (duh) but I can sell you that if you like, email me about it if you are interested. You provide the power supply, of course...

27th July 2007, 10:19 AM	#309
rjm Richard Murdey diyAudio Member Join Date: May 2004 Location: Kyoto Blog Entries: 72	For a number of reasons its a good idea to leave R1 as 47 kohms, as a general rule that's pretty much what the designers of step up transformers expect to be loading the output.