Thank you for this suggestion. I would be very grateful if could give a bit more detail e.g. possible values of resistances, number and circuit arrangement.
Hi,
The resistor divider idea is fine except you need to be careful about the "balanced 600-ohm input" to the PA. This may use a centre tapped input transformer to give high common mode rejection of noise (e.g. mains "hum" pickup). If so the centre tap will be connected to ground.
Using an unbalanced output earphone socket from your tape recorder may mean one side is connected to ground at the tape recorder (if it's floating from a transformer secondary that's fine).
If the output is not floating then when you then provide the resistive divider/attentuator one end must also be at ground, and connecting one input of the PA to ground to complete the circuit would would also short circuit half of the input transformer winding. This would cause severe, if not complete, signal loss to the PA, probably with high distortion as well.
Thank you for this very informative and very interesting analysis. I will reflect on what you say -- and do some background reading on balanced circuits.
The tape recorder is battery operated so that simplifies things -- doesn't it?
Hi Bob B,
Yes you are quite right, if the tape recorder is battery operated then it should not be a problem as the case (plastic?) will insulate it from ground.
In an earlier posting you asked about resistor values. Do you know what signal level the PA requires? It may be relatively insensitive needing about 0.5V signal to get full power.
If your earphone output is of the type which diconnects the internal speaker when you plug it in then the signal level could be of the order of a few volts. In this case a simple divider could consist of a series resistor of perhaps 2200 ohms (2K2) and a shunt resistor across the PA input of 620 ohms (620R).
If you don't get enough signal to the PA then reduced the value of the series resistor a little at a time. The best thing to do is experiment! You could alway use a potentiometer (say 5K) wired as a simple volume control to get the optimum setting.
So I see that the input you intend to use is actually intended for a dynamic microphone. (This fact means that it is very likely to have the centre-tapped transformer that I mentioned earlier as this is very common on microphone inputs)
I don't know the particular model of Shure mic (makers of an excellent range of audio products) but the input sensitivity is going to be millivolts - not volts. It would be better if the PA had a less sensitive input - is there one marked "Line" or "Aux" ?
If you have to use the Mic input you will need to attenuate the output from the tape player a lot more than I first suggested, so increase the series resistor to at least 22,000 ohms (22K). It may require to go even higher , say 47K, to avoid overloading the input stage of the PA. (This would be evident by lots of distortion regardless of the setting of the PA volume control, as this is usually after the Mic pre-amp stage)
If you get the signal level to be acceptable without distortion, then on a purist note you will find the frequency response a little odd: excessive bass due to the Mic input compensation circuit and loss of treble due to our external resistive divider. This can be corrected, however it will need a few more components!
Regarding frequency response, from what you say about the amplifier it probably has just a simple "flat" Mic input with no frequency compensation, so ignore my previous comment about excessive bass response.
The "cartridge" input may be a better bet if it for a general purpose piezo-electric (crystal) type as some of these can have quite high output voltages (250-500mV). If it's for a magnetic cartridge this will be less useful as it will be like the Mic input for low level signals, and the RIAA frequency compensation will be wrong.
The reduced treble response I referred to could arise if you need to use very large values of series resistor to get sufficient attenuation of the signal for feeding the Mic input. This is because the various "stray" capacitance to ground, including the input capacitance of the Mic input, make an unintended simple low pass filter (i.e. a "treble-cut" tone control)with the series attenuator resistor.
This problem can be corrected easily by adding a compensating capacitor in parallel with your series attenuator resistor which is equal in value to the stray capacitor values. As these are likely to be unknown try a few values between perhaps 100 and 500pF until it sounds right!