Preamp gain question

Ouch! 80db of gain in the preamp, with a 6.5mv cartridge. I wonder if 10db attenuators will be enough to get the volume control into the optimal part of its range, where resolution is good, and perhaps even into the part of its range over which it can maintain channel balance accurately as volume is changed.

80db means a voltage gain of 10,000, so that if the cartridge is putting out 6.5mv (at 5cm/sec@1kHz, if I remember correctly what the conditions are at which cartridge outputs are normally specified), the preamp output with the volume control at max would be 65 volts!

Maybe 20db attenuation would be more appropriate. And I'd also suggest checking the specs on the preamp, particularly its maximum rated output voltage, to be sure it is really suitable for such a high output cartridge. My concern would be that with the volume control set say in the middle of its range, at 12 o'clock, that high amplitude signals would get clipped before they even reach the attenuators.

Regards,
-- Al

Photon46- what would be an ideal amp/preamp gain be? Are you saying that they should be more closely matched?

I think he's just saying that the overall system gain of 110db in phono mode is simply too high, particularly for a high output cartridge such as you have.

What is meant by a better match is that a very high gain preamp should be mated with a relatively low gain power amp, or vice versa, so that the overall gain doesn't become too high in relation to the cartridge output.

Hopefully the attenuators will accomplish that, although as I said it is conceivable to me that 10db will not be enough attenuation to be satisfactory. And it is conceivable to me that you will still be limited in how far you can turn up the volume control by how much voltage the output stage of the preamp can swing, which would not be helped by the attenuators because the attenuators would be connected "after" that point (further downstream, so to speak). I'm not familiar with that preamp, but the spec on output voltage range might give some feel for that.

Regards,
-- Al

I looked at the specs at the CJ site, and output voltage range is not specified. Oh, well. But it's interesting to note that in the PV10A they reduced the overall gain by 13.5db, relative to the PV10.

Regards,
-- Al

The 10A is linestage only, could that be why?/

According to what I read at their site, the 10AL is linestage only, while the 10A includes a phono stage. Although it was in the line stage of the 10A that the gain reduction was made; apparently an identical phono stage was used on both the 10 and 10A.

I take it that changing tubes is not a solution?

I don't think so. A good circuit design is designed to be as independent as possible of variations in tube characteristics, within the range of variation that can be expected for the specified tube type. Although in a no-feedback design such as this there will probably be somewhat more variability as a function of tube characteristics than with a design that uses a lot of feedback. But I still don't think that is the way to approach the problem.

Regards,
-- Al

I wanted to add the following sentence at the end of my previous post, but we don't seem to have an edit function on this forum (someone please let me know if there is a way of editing one's own posts):

A tube set for a preamp that would result in a 10 or 20db reduction in gain from the design's specifications is probably defective.