preamp out instead of MC transformer

It is not even wrong, as Wolfgang Pauli use to say. A preamp output would hideously overload a MM input. I thought I had heard every strange idea in audio; I was mistaken. The good news is that this is a strong " worst idea of the year" candidate. I don't know enough electronics to fully describe why this would not work.

Not sure why you said "back into the preamp in." To have any chance of working, it seems to me that the output of the second preamp would have to go to the power amp.

Given that, while it is conceivable to me that the idea might work with certain components, in general there are all kinds of things that could go wrong. Depending on the specific designs significant concerns would be that dc offsets, turn-on transients, and/or turn-off transients produced by the first preamp could be amplified by the second preamp to potentially damaging levels. Low level hum that might arise due to ground loops involving the two preamps could also be amplified to very high levels.

If all of those problems could be avoided, it is probably more likely than not that the signal-to-noise ratio of the first preamp, with respect to its line-level inputs, and/or the gain of that preamp, would be too low, resulting in unacceptable hiss levels.

All in all, it might work under some circumstances with some components, but I don't think it is a promising path to pursue.

Regards,
-- Al

It is likely that Al is correct and the gain would be far too low. In my first response I had assumed that it might be possible; however , even if it were remember that the output of a line stage is measured in volts and the input sensitivity of a phono stage is measured in MVs so there is a probability that you will overload the phono stage. Considering the cost of extra cables and preamps a step up would be much cheaper and far easier to work with. Bob's Devices is a very good one at a moderate cost.

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.

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.

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.

Equalizers and other processors were, and sometimes still are, generally inserted into tape loops, which places them "before" the volume control. In some situations it might also be appropriate to insert a processor between preamp and power amp, or in the case of an integrated amplifier between the output of the preamp section and the input of the power amp section. That of course would put the processor "after" the volume control.

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.

If the MM phono preamp is inserted into a tape loop of the other preamp, as I indicated earlier preamps often do not provide much if any gain between their line-level inputs and their tape outputs.

If the main output of the first preamp is connected into the MM input of the second preamp, then as you now appear to realize the output of the second preamp would have to go to the power amp, not back to the first preamp.

That arrangement would function, but it would also create the risks and sonic issues that I described in my first post above. As well as the loading issue that Armstrod pointed out.

The same cumulative gain could be realized, without most or all of the risks that I described, by simply connecting the turntable into the MM phono stage, connecting the output of that preamp into a line-level input of the other preamp, and connecting the output of that preamp into the power amp. The result, though, would probably be totally unacceptable hiss levels, because chances are that the signal-to-noise ratio of the MM phono stage would be inadequate for LOMC's.

Onhwy61, thanks for the nomination!

Regards,
-- Al

Seems to me, your question is not "stupid", but there are some factors to consider.

In concept, you just want a buffer amplifier with about 20 dB of gain (because that is the average difference bewteen MM outputs and MC outputs, a factor of ten) to put between the MC cartrigde and your MM phono input.

In theory, a line stage amp will give you this, depending on the design, and you would probably need to use its power amp outputs, because line level outputs like TAPE out will not have enough gain.

But, in reality:
(1) You need a second linestage amp to go from your phono preamp to the power amplifiers, unless its already got a gain control built in;
(2) You cannot use the same linestage amp for both functions as explained above;
(3) You will have noise problems because the noise floor on most linestage amplifiers is much higher that what is desirable for MC inputs;
(4) You cannot load the MC cartridge correctly if it is a cartrigde that requires a low input impedance such as 100 ohms. That being said, there are some high output MCs that can be loaded at 47K.

So if you have two linestage amps of execptional low noise performance, its possible, depending on the cartridge. But probably not practical.