preamp out instead of MC transformer

Almarg,
The idea of running it out of the preamp out to the mm preamp and back into the preamp in of the same preamp is to use the MM preamp like an outboard processor, much like people used to do with equalizers and such in the past. I didnt meann to imply using two line preamps.

I did questioned whether the phono input would be overloaded. When you have some extra stuff you start wondering about novel ways to put it together and either fry something or come up with something useable. A little knowledge is a dangerous thing.

The idea of running it out of the preamp out to the mm preamp and back into the preamp in of the same preamp is to use the MM preamp like an outboard processor, much like people used to do with equalizers and such in the past. I didnt meann to imply using two line preamps.

I don't see how that could even give you a complete signal path. If you have only one preamp, its input select switch will only be able to select EITHER one of its line-level inputs, such as Aux, OR the MM phono input, not both.

If you were to connect the tape output to the MM phono input, you would have to set the input select switch to Aux to get the signal to the tape output. Therefore the output of the MM phono stage would go nowhere. Also, many preamps don't provide any gain between the line-level inputs and the tape outputs.

If you were to connect the main output of the preamp to the MM phono input, and select the Aux input, again the output of the phono stage would go nowhere. If you were to select the MM input instead, with the volume control turned up significantly, you would essentially have put a feedback loop from the output of a high gain amplifier back to its input. A high volume oscillation would occur, that stands a good chance of being destructive.

Apart from the oscillation scenario, btw, it is unlikely that overloading the MM phono input would be among the problems that would be faced. Most preamps provide overall gains from their line-level inputs to their outputs (with the volume control turned all the way up) that are in the rough area of 10 db to 20 db or so. 10 db corresponds to a voltage gain of about 3; 20 db corresponds to a voltage gain of about 10. So the output of a 0.5 mv cartridge would only be stepped up to roughly 1.5 to 5 mv.

So the bottom line, as I see it, is that using one preamp simply won't function, while using two preamps will function but probably in an unacceptable manner.

Regards,
-- Al

It's not a stupid question. Actually I think it's quite inventive.

Almarg should be nominated for sainthood for taking the time to give such a detailed answer.

I don't see gain/overloading as the biggest issue here. Head amps generally add 20-30 dB of voltage gain, and there are many line stages that add 20dB and a few more than that. The real issue is proper resistance loading of the cartridge. While you could use Y connectors to add a loading resistor, it would be in parallel with the input impedance of the preamp. This may or may not be a known quantity, and might vary greatly with frequency. Real head amps are designed to minimize these issues.

As long as you keep the volume down on the initial run, I doubt you can hurt anything by trying. If you've overloaded at any point in the chain it will become obvious in short order.

Ok, in my scenario, there is a line preamp and a MM phono preamp, 2 seperate preamps. In the old days, signal processors where used by hooking a loop into the line preamp. However, now that you mention it, I dont remember if it was from the preamp out to preamp in, or record out to record in. I am envisioning the MM phono preamp in the same location as the processor would have been, mainly to add RIAA equalization and gain. So the line would go from cartridge to aux input to line preamp out to MM phono preamp in to mm phono preamp out to line preamp in.

Actually, as I wrote this I realized I was thinking of an integrated amplifier that had a preamp in. I guess I would have to go from the MM phono preamp out to the amp.

The idea is to get the line gain and the mm gain to cumulatively be enough for a MC cartridge, but writing this has made me tired. too tired to think about it anymore.