I have a small audio amplifier project I am working on.

I've built and tested the pre-amp I bought from Amazon.

You can find what i did and what I thought about it at:

 Rohde & Schwarz MXO4 Scope meets Thermionic valve pre-amp 

How are you getting on with it and the power amp ?

MK

I wired up the transformers, the inputs and outputs.  

The output of the preamp into an 8 ohm speaker is barely audible.

If I feed the pore-amp into the power amp, there is absolutely no output.

I have checked the voltages at the power in to both the pre-amp and the power amp.  One thing I noted is that there are two places on the power amp that are labeled J1 and J2.  There was no mention of them in teh so called instructions.  I worried that these were for some kind of option.   I will jumper them to see if that makes a difference.

I am not surprised about the pre-amp.  I may look into trying to improve it or maybe just junk it.

Roger

I think the jumpers need to be fitted.

Do you have any test gear like DMM, Scope or signal generator ?

If it were me I would start it up with a 33R 5W resistor in series with each transformer winding - that way it can't draw more than about 0.5A but would just about work.

Best not to start with  decent speaker connected - a fault on this kind of amplfier can apply DC to the speaker and burn it out.

What would your ideal spec for a home build pre-amp be ?

MK

MK,

I have a very nice Sony home theater system with some (pretty good) Polk floor standing speakers, with sub and surround.  I have it set up in my 'media room'.  I am trying to set up some quality audio in my office.  I am retired and have been delving into electronics and physical computing for a few years.  I have built loudspeakers in the past.  Right now I am trying to keep my mind stimulated with the learning of new things.  My forte at work was solving technical problems related to polymer science in the plastics industry.   I am involved heavily with the home care of my wife, and thus have to occupy myself with something indoors or go crazy!

My objective is to try to build a few simple and inexpensive projects to get my feet wet.  Eventually, I hope to build a Blutooth system to my office from my HT system, and play desktop music through a home built amplifier.  Whether this requires a pre-amp or it can be an integrated amp.  I do not at this time expect to need to select between front end sources so a traditional pre-amp is not necessarily required.  At some point in time I might like to start back up with vinyl, which would require perhaps a phono stage.

All in good time.  I am not looking for the ultimate anything right now.  I started playing with 'record players' taken from the trash on my way home from school some 60 years ago!

I hope to build it as a single ended tube style amp, with perhaps 5 or so watts.

I DO have a DMM but no scope.  I am looking at a few options like a bitscope or other low cost system, one that uses a PC sound card is interesting.  If I ever get really into this, I would consider upgrading.

Roger 

MK,

I have wired up actually re-wired up, with secure connections the audio source (AV jack from RPI) to the input of the pre-amp, the pre-amp to the power amp, and the power amp to the speakers.  The anmps are powered by separate transformers off of the load and neutral of my mains through an IEC connector that iws switched and fused.

There is no 'ground' on either transformer.  The center tap on the 18vAC CT transformer is connected to the 'ground' connection of the power amp.  Lots of hum when just powered up.  I connected the neural on the pre-amp to the center tap of the other transformer and then checked continuity.  The neutral on the pre-amp is 'closed' circuit with all of the 'grounds' of the input and output jacks on the pre-amp, with the input ground on the power amp, with the ground on the transformer center tap with the power amp, and with the neutral or ground on the speaker outputs.  When I turn on the power to the whole system, with the power switch on the pre-amp in the off position there is hum from both speakers.  When I turn on the pre-amp, the hum gets much louder.  When I play an audio signal into the pre-amp the audio comes through on top of the hum.  The hum and audio are approximately equal in volume.

I disconnected the power amp from everything, including mains, and connected the speakers to the pre-amp, and got some hum and very weak (of course) audio from the pre-amp.  I am uncertain how to approach the grounding issue.  The mains ground is not connected to anything.

Still a work in progress.  Going to try to use just the power amp, or just the pre-amp to see which one or both are responsible for the hum.

Roger

I like playing with audio but my playing time is limited. I'm scheduled for a knee replacement op soon (might know better when next week) so I'll have some forced time off.

Designing some audio stuff might be fun. I have 100 of these op amps - ideal for audio low signal work:

OPA1655:

Posted this a couple of days ago but forgot to click "Reply" !

MK.