My problem... I've got 2 preamps, but neither of them individually has enough aux/accessory inputs to handle everything I want to connect to my power amp (TV, computer, tuner1, tuner2, CD deck, tape deck, DVD player, VHS player, etc.) My amp has only a single set of inputs. Can I safely connect the outputs of more than one preamp to the inputs of my power amp, and if so, do I need to isolate the preamps from each other, and if that's the case, would I just use a pair of diodes set up like a bridge rectifier?
Yes, the first unit becomes the slave and the second will be the master which will control volume and all other functions of that pre amp. No need to set the volume to any point on the slave as tape out is unaffected by the volume (or any other) control beyond the selector switch. The master then feeds to your power amplifier.
Thanks for all the excellent info and suggestions. As is the case with all newbies, I have yet two more questions:
1) (for Mike3) How do I identify/measure V-out of the preamps and the amp... I'm guessing tip to ring of the line out of the preamps, and between + and - speaker output on the power amp? Am I looking for voltage similarities at any particular volume setting, which leads to question #2 below...
2) Does output voltage remain constant with variable current when operating the volume control pot? Obviously, I've barely scratched the surface of this technology yet.
Provided the items aren't out of the ark (i.e. pre 1970), any line level input should be compatible with the tape out of the slave preamp. When I say line level, I mean anything which isn't marked Phono, such as AUX, Radio, Tuner, Tape, CD...
Gotcha. Unfortunately my setup is pretty creaky old stuff. Not sure I can track down the spec sheets but I'll try. If I can't, is there a way to check actual values with a multimeter?
Tape out provides a line level voltage and will work into virtually any line level input. However, the reason I suggested using the tape input on the master unit is due to the typically higher parts count in the circuit of inputs such as Tuner and Aux. By going into the Tape In you will more than likely minimize the circuitry the slave pre amp runs through. This will impart less of the sound of the master to the the final product and minimze any incongruities between the two pre amps. This also works well for CD players, run your player into the tape in of the master, if you have a second open tape in, and you will almost always clean up the signal vs. running into an Aux. input. If you decidely prefer the sound of the master pre amp, run the slave into a line level input, otherwise, try the Tape In and listen for any perceivable improvement in clarity from the components plugged into the slave.
If you have used both pre amps with the current power amplifier without experiencing problems concerning level, I wouldn't be too concerned with measuring the output voltage of the pre amps. If one pre amp was intended to run with the power amp, make it the master.
Interesting - I thought it was the other way around, that the Tape input had the higher component count since both Tape In and tape Out have extra buffers over and above the circuitry of the standard line level inputs. Boy did I get that wrong!
You guys are awesome. Thanks for all the help! You've given me everything I need to git'er done.
I still am wondering about what standard line level voltage is typical (6, 12, 48?) and whether or not it varies or stays constant and the current varies with volume adjustments. No worries, though. If I don't hear back, I'll just get out my meter and fiddle about.
Plus, this bunch is not only technically savvy, they're funny enough to get me to squirt coffee out my nose this morning... "Beethoven's head on a stick or somethin'," for a commercial audio test unit. Yeah, baby! Gotta get one of those.
In consumer audio gear there is no standard for anything. Most line level voltages prior to CD ran about 1-1.5 volts. CD players introduced a 2-3 volt level which overloaded many older pre amp sections. Some players were capable of even higher output. But that is only on peaks, so the average level falls far below the peak potential. However, if you hear a crashing, distorted sound on peaks when using a CD player, the first suspicion would be front end overload of the pre amplifier section.
Current is not as important in line level input/output as would be voltage but current delivery will be dependent upon matching the impedances of the source to destination pieces. Low output impedance running to high input impedance will typically give you all the current you require for line level with a 10X out-to-in ratio suggested. The lower the output impedance of any line level source, the longer the cable runs can be without seriously affecting the sound quality, asuming, of course, the input impedance of the receiving unit is sufficiently high.
Frank - Many pre amp line inputs will have a buffer on their source side to prevent the input impedance from rising with the position of a potentiometer type volume control. Most tape ins will be minimal circuitry since they do not run through the volume control at least. Tape in/out may have a buffer but probably not in consumer devices since the impedances of connected devices are not subject to the vagaries of the VC position, are somewhat consistent from unit to unit and cables to and from such devices are usually kept short.
Many pre amps will have output buffers to once again lower the output impedance of the pre amp and maintain a consistent impedance as volume changes. It is a trade off that high end designers sometimes eschew preferring instead the minamalist approach to less circuitry in the signal path. However, it is difficult to get discrete component circuits to have a very low output impedance without some sort of output buffering (almost impossible with a tubed pre amp unless cathode followers [or similar connections] are utilized which still amounts to more circuitry). If the designer chooses to forgo the low output impedance of a buffered output stage, the owner of the device should be made aware of the restrictions on cable type and length. Partnering with a high input impedance power amp would be the wise choice but will not eliminate the need for caution in cable choice.
I thought 500-600 Ohms was the generally agreed upon low impedance point for most pre amps when I was selling. This would normally be sufficiently low to drive 200' of low capacitance cable without loss. However, I have recently seen pre amps which boast a 50 Ohm output impedance.
Add to the above, a line input will typically run through some amount of gain circuitry whereas a tapein/out is functionally kept at the same voltage/current from in to out and back again. Gain requires either more circuitry or a transformer.