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Phono preamp, help.

S

SFM-Ptgal

Something that I noticed a long time ago. I bought and assembled this kit to connect to the Stereo 80. I connected it to one of the TT inputs. The level is too hot so it needs to be padded. The att I should give is unknown for me. The clue is that for an average level of 0db the pot must be between 2 and 4 (yes, 2-4). I'd rather prefer to change the circuit to make the att at the output. I noticed also that the audio is too treble, comparing with other sources (same song, different sources). The pickup is a sc35c.
What can I change (caps for eq and resist. for att) on this circuit? http://www.velleman.eu/downloads/0/user/usermanual_k2573.pdf

Tks!
 
Without downloading the 5.6 MByte manual I might suggest checking the equalization to make sure you are using the right equalization (e.g. RIAA) and the right cartridge selction (magentic vs ceramic). Too much treble is generally the wrong equalization curve and too much output, the incorect type of cartridge selection.
 
Looking at the manual, it is RIAA eq curve and is for a magnetic cartridge input.
Which the Shure SC35C cartridge is.

First, I would listen to the output of the preamp with headphones and see if
it is still too treble. If so, I would suspect one/all of the electrolytic capacitors
on the preamp. If it is older than 6 years, I would change them anyway.
I would also scrutinize the proper placement of components on the circuit
board since you assembled it. A couple of similar looking but different resistors
or ceramic capacitors may be reversed. This has happened to me before, as I am
sure others when assembling KITS. This woud effect the EQ curve.

If it sounds correct in the headphones, I would suspect the input channel on the Stereo 80.
If you haven't already, move it to another input on the console.

As far as attenuation, I would do it between the pre-amp output and the console input
with a standard resistor attenuator.
 
Try hooking the preamp output into one of the other inputs of the Stereo 80.

Been a long time since I've even seen a Stereo 80, but you said you used a "TT pot."

Perhaps the console has it's own built-in phono preamp--this would explain both your gain problems (since the built-in preamp would be presumably designed to connect directly to the phone cartridge) and EQ problem (since it would not be flat but would have a standard RIAA eq).

Don't remember this particular Shure cartridge, but Shure phone cartridges were contemporaneous with the '70's vintage Stereo 80. Model #44 comes to mind out of the murky recesses of my memory.
 
I don't recall the Stereo 80 having any phono preamp cards in it. A mic preamp card would exhibit high gain and excess treble, but it would be mono. It seems that console had Cart 1, Cart 2, Cart 3, TT1, TT2, Remote 1, Remote 2, etc labels on the input select switches but IIRC, they were all high level inputs and the labels were just a guide in case you wanted to follow the Harris suggested layout. Thing I recall about the stereo 80 is that it was rough on electrolytic caps in the program circuits, often resulting in a loss of low end. The untrained staff would often turn one channel's master gain up to compensate for the seemingly lower gain with the result being that you could "see" the treble notes clearly on the VU meter.
 
No, no phono preamp. I made some tests: I inserted the TT preamp out on a tape input. Besides the level (too low on the tape input) the eq. remains the same. A CD player doesn't show any eq differences, nor the PC. This leads me to conclude that it might be a circuit concept problem. This eq. problem occurs just with the TT preamp.
 
That would put the problem back with the preamp. The level problem might be curable--perhaps the console has a pad that can be changed on the TT channels.

The preamp should have a pre-emphasis circuit that compensates for the RIAA curve, perhaps it is not in the circuit?
 
Couldn't get the manual to download before--funky link, so I went to the site.

Not a design expert by any means, but this circuit just doesn't look like it has enough to it to work right. TLO72 run single ended, 1 uf caps in and out, unbalanced in, of course, but also out, as they are using the dual op amp for stereo. I would like bigger coupling caps say 47 uf, for one thing. Nor any means to control output level, probably because they are using the feedback to attempt to equalize the circuit. Normally the feedback resistor value sets gain.

Maybe someone has a better circuit...
 
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