Question about M7 Preamp

[vangogh-hk]

I have complete this preamp as shown in.

http://diyparadise.com/simplepreamp.html

I connect this to a 300B power amp.

But I found that when the volume in the preamp turn to above (30%)10 o'clock, the sound come out too loud and start to distorted. This means that the range above 30% of the volume is useless.
Should I replace the volume VR with another (ohm) one or adjust the value of other component(s) inside the circuit so that I can have a larger range of volume?

Thanks for your help?

 

[Audiofool]

Try to remove the cathode bypass cap : you will have less gain and get rid of the negative effect of an electrolytic cap in the signal path.

J.B

 

[Burnedfingers]

How efficient are your speakers? Could it be that you are clipping your amplifier because you are trying for more level than the amplifier is capable of?

I have also built this preamplifier and have run it with my Quicksilver Mono block amplifiers and my Dynaco Stereo 70 without problems.

 

[tubemaster]

Hello,

try removing the preamp and connect the source directly to the 300B amp.

Best regards
Michael

 

[vangogh-hk]

My speaker is 90db 4ohm Blue Room mini-pod.
I also think that it is due to clipping.
Although I can bypass the pre amp. But I found that with the preamp added between the CD and power amp, the sound image is wider and deeper.

I know that eliminating the cathod bypass cap is a solution, but I heard from other reference that:
"eliminating the bypass will result in the cathod being not a signal ground, making it more easy to pick up noise from heater's magnetic field.
Besides, series -ve feedback will increase input impedance and affect higher band frequency response at the coupling stage"

Is it true??

Can I adjust the loading resistance to adjust the gain?

Thanks

 

[analog_sa]

Excessive gain is the eternal curse of active non-nfb tube preamps. And it's not easy to get rid of that gain without screwing up the sound.

The only elegant solution to your situation (unless you are willing to adjust the output of the dac) is to use a step-down transformer - either replacing the plate load or 'parafeed'. A cheap Ludahl 1660 sounds very nice connected as 4.5:1

 

[tubemaster]

Hello,

Although I can bypass the pre amp. But I found that with the preamp added between the CD and power amp, the sound image is wider and deeper.

OK, that seems like a good reason to use it. 🙂

I know that eliminating the cathod bypass cap is a solution, but I heard from other reference that ...

In practice removing the cathode cap from a prestage often results in a better sound. Just try it there is no danger in doing so and it will reduce the gain notably.

Also you migth want to check your cathode voltage to determin that it's not the 5687 which is clipping. Which version of the preamp did you build, i.e. what is your load resistance, cathode resistance, cathode voltage, b+ voltage and anode voltage? How much voltage swing does your 300B need for full power output?

Best regards
Michael

 

[vangogh-hk]

The pre amp I built is in

http://diyparadise.com/yeo5687.gif

The power amp I use is 6k7G Capacitor couple 300B

http://www.bezdz.com/images2/T3B.jpg
Specification is:
Sensitivity=0.7V
input inpedance=100K

It seems that sensitivity is too high to add a preamp, but I don't want to give up the improvement in sound image brought by the preamp.

 

[tubemaster]

Hello,

did you measure the voltages in your circuit to confirm that they correspond to those on the schematic?

If so, try removing the cathode bypass cap.

If the gain is still to high you can insert a high quaility 100K resistor before the pot (avoid metal types). This will reduce the input signal by 50%.

But measure the voltages first!

Best regards
Michael

 

[kmtang]

Make sure that whether the distortion happens in the line amp or the power amp.

It could be the power amp drived into clipping state.

Johnny