Legacy Preamp Schematic

Posted by coffee-phil (A) on September 2, 2010 at 18:25:26

A couple of months ago I posted some pictures and the description of my legacy record preamp and equalizer with the promise that a schematic will follow. As I said at the time if you are listening to mono records with a stereo cartridge you need this or something like it. Also if you have pre RIAA equalized records something like this is very useful. I have finally compiled all my notes and scribbles into what I believe is an accurate schematic of what I built. I have not included the power supply as I worry about liability for anything which plugs into the wall. I also figured that anyone who will attempt to build this should have little trouble coming up with plus and minus 8 to 12 Volts at less than 50 ma.
This preamp facilitates the use of a stereo phono cartridge for playing mono records which are either laterally or vertically encoded and offers an array of base turnover and treble cut frequencies.

Brief description of the circuit:

The signal from the phono cartridge enters at J1and J2 and is passed to the function switch SW1. The first position of the switch is the stereo position which routes the signal out J3 and J4 to a stereo preamp. The second position is the left position which allows the preamp to see only the left channel of the cartridge. The next position sums the two channels. The channels can be summed in phase for lateral encoded records or out of phase for vertical encoded records. The fourth position of the function causes the preamp to respond to only the right channel of the cartridge. Rumor has it that one side of the record groove can be damaged while the other side is still useable. I have yet to find a situation where left or right only is useful however it didn’t cost much so I put it in. When in the mono mode the cartridge is loaded with C1, C2, R2, & R8. Those values can be customized to the needs of your cartridge and cable.
U1A and U1B are the input amplifier buffer amplifiers for each channel. They each provide about ten dB of voltage gain.
U2A is the summing amplifier as well as the bass turn eq amp. The voltage gain of this stage is about 10 dB at one kHz at any bass turn setting. For lateral records the outputs of U1A and U1B are summed. For vertical encoded records the left channel signal from U1B is routed through a unity gain inverting buffer U2B before being routed to the summing node at U2A. Lateral or vertical encoding is selected with SW2. The base turn frequency is selected with SW3 which switches in the appropriate feedback network for U2A.
The output of U2A is passed to the treble cut network R27, R28 and the capacitor selected with the treble cut switch SW4.
U3A is the output amplifier. The gain of the output amplifier is about 10 dB.

Some notes may be useful:

I have bypassed the supply of each of the op-amps with 0.1 microfarad X7R ceramic capacitors. As an RF person it is my practice to bypass all of the supplies of my active devices well. X7R is good enough for supply bypass.

All of the capacitors in the signal path should be some decent film, COG ceramic or mica.

Note that the values of the capacitors in the eq networks (C13 through C25) are not standard values. Chose standard values below the indicated values and add additional smaller capacitors to make up the total value within 1%. Use a capacitance meter.

There are flat positions in both the treble cut and bass turn controls. If these positions are selected the response is constant velocity (with a magnetic cartridge). This is appropriate for acoustic recording however some treble cut will reduce record scratch. Some bass turn may also sound better. For early records you may have to experiment a bit. Setting both the bass turn and the treble cut to 700 Hz will give a constant amplitude response.

SW3, the bass turnover switch should be (in fact I almost want to say must be) a shorting type switch. These are also called make before break switches. The reason for this is to insure that the feedback around U2A is not removed while switching from one bass turnover frequency to another.

I do plan to copyright this schematic however if you want to build it for your use I would be pleased. Do post pictures and report your impressions of it.
Naturally if you want to manufacture and sell them we need to talk.

I have linked my old post with the photos.

Phil

http://db.audioasylum.com/cgi/m.mpl?forum=vinyl&n=916343&hig (Open in New Window)

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Topic - Legacy Preamp Schematic - coffee-phil 18:25:26 09/02/10 (6)

Phil, can you - 1973shovel 19:17:37 09/02/10 (5)

RE: Phil, can you - coffee-phil 22:32:17 09/02/10 (4)

Anyone? Even me? - Muzikmike 05:45:37 09/03/10 (3)

RE: Anyone? Even me? - coffee-phil 10:31:55 09/03/10 (2)

It's only $75 per month! - 1973shovel 10:54:09 09/03/10 (1)

RE: It's only $75 per month! - coffee-phil 11:26:31 09/03/10 (0)

Legacy Preamp Schematic

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