Marantz 7 preamp inspired clone issues

[ee.jcesar]

Hi,

I am building a Chinese kit for a friend, I have no experience with tubes or audio so this is quite challenging to debug.
This is almost the same kit that my friend got

https://www.fyfaudio.com/products/h...amplifier-marantz-m7-circuit?VariantsId=10237

The wiring diagram is not very clear but I managed to understand the connections, it doesn't work, the output volume is very low.

I measured all the voltages (top caps and pin 1,6, 3 and 8 of tubes) and are within a 10% range of the values of the schematic above, what would you think is wrong?
it seems that is rated 30W, I am using a 6ohm 10W speakers, audio is very low, doesn't sound distorted to me
Thanks

 

[jjasniew]

So you're aware this circuit isnt intended to drive speakers directly? The output from the far right 12AX7 is supposed to go to a power amplifier, then from that device to the speaker. The 30W rating is the power it takes to operate, not the power it can provide into a speaker load. Hope this helps!

 

[huggygood]

 

[ee.jcesar]

Oh boy,

Thanks jjasniew

 

[ee.jcesar]

on a second thought, I measured the output at about 100mV with a full input volume of my phone (sinusoidal signal, 1.7V peak)

to be fair the input frequency was quite low, I think it was 200Hz, without a frequency response graph I can't know if this is OK

 

[jjasniew]

Oh boy,

Thanks jjasniew

I heard the story of a Chinese fellow visiting guitar amplifier forums. It was clear he had some learning to do at the time. Today, he has his own line of amplifiers in production, all - apparently - based off what he learned participating on line. I predict you'll be there soon.

I had a real Marantz model 7 when I was a teenager - lost it in a stupid trade in adulthood. I'm sure the one depicted in the above schematic is going to sound pretty good, properly applied as a preamp. Next challenge will be to find a matching (by way of sound) power amp. https://www.thanksbuyer.com/tda7294-2-x100w-current-feedback-audio-power-amplifier-board-27331 Current feedback is supposed to sound more "tubey" and I'm sure the 7 was intended to match up with Marantz's all tube power amplifiers.

May I suggest getting and learning REW. This software can do an oscilloscope display, a frequency response plot and harmonic distortion analysis. The "box" to get the audio signals into the computer might be a little tricky with those signals riding on 150V DC, but it can be done. With this, you should be able to go through that circuit - with the signal input from your phone - point by point and see what's there - or not.

At this point it's anyone's guess as to what could be wrong, as 1.7V in 100mV out doesnt sound quite right. Maybe the 75k got connected to the wrong 1k - there's three of them in the vicinity.

 

[ee.jcesar]

Thank you, I appreciate your response.
I am very comfortable working with higher frequencies, antennas, digital signals etc but audio has never been in my radar before. I have to admit that the first time I turned on the unit I didn't expect to be that hot

Thanks for the REM recommendation, I might be able to use it at work
Cheers!