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The Newcomer's Forum / Re: Peavey Audition Plus
« Last post by willpirkle on October 16, 2019, 03:49:24 PM »
Agree with previous reply - if you are new to this, I'd definitely try to get the correct schematic as well. Peavey customer service (at least here in the states) is usually very generous with information and help. The last thing you want is to be second guessing the components, part numbers, etc...
The Newcomer's Forum / Re: Peavey Audition Plus
« Last post by DrGonz78 on October 16, 2019, 03:43:19 PM »
Look inside the amp chassis and confirm if there is a 2030 type opamp power IC. Not sure how closely related your amp is to the schematic posted but we'll find out as we go. Start by confirming the 2030 and then we're off to the races.
The Newcomer's Forum / Peavey Audition Plus
« Last post by psafloyd on October 16, 2019, 12:22:33 PM »
The prelude
I have acquired one of these amps that has a particular problem. Having searched the site, I realise that others may have had a similar problem and I am hoping someone might recommend a sensible plan of attack.

The problem
The amp is in lovely nick, but on firing it up, there is a loud steady hum. This hum is not affected in any way by adjusting the volume or any of the controls, including the push button channel switch.

On reading previous posts, I checked if the speaker moved in one direction on start up and it does. So, I measured for DC at the speaker and this came out at 9.5V.

I haven't had it apart yet, as it only came in today, but am hoping for a plan of attack. But if there is DC at the speaker, this points to one of a number of potential causes, and I wondered if someone might suggest which to tackle?

I cannot find the Audition Plus anywhere as a schematic, so wonder if the difference was just branding. I have attached the Audition 20 schematic.

A whinge about the paucity of adult education

And if anyone is wondering why I haven't learned more since my first posts, I have been trying. I even tried to find a part time/evening course in electrical engineering or electronics and its a nightmare.

There are no electronics ones any more – I KNOW there used to be. As for the electrical engineering ones, they are either long vocational courses, or do little more than qualify the individual in PAT testing.

If anyone knows of any in the east London/south Essex areas, I'm all ears. Or a good correspondence course.

The thanks
Thanks in anticipation.     
The Newcomer's Forum / Re: Carlsbro Hornet (1970s)
« Last post by psafloyd on October 16, 2019, 11:37:28 AM »
As I was coming on for the first time in a while, I thought I'd post an update.

To all those who offered assistance, many thanks. After changing all the electrolytic capacitors, the amp was working far, far better, with a more noticeable wobble in the tremelo effect.

It's still not perfect, but I had to put it aside when I moved my office and I plan to go back to it this winter to see if I ca improve it and put it into a basic cabinet.
Same hybrid ckt but high level out.
With a push-pull circuit the same triode produces almost no even order harmonic distortion at all, as usual.
That is true, until you start drawing grid current on the input side like when you overdrive a power amp. Depending on the source impedance, odd harmonics will drop out.

See experiment with TDA2040 here:
Any asymmetrically distorting circuit will produce similar profile. E.g. single-ended common emitter amp.

And no, triode (in a specific circuit) won't do it across a wide range of output levels either: With small signal input the magnitude of distortion is negligible, as usual. With modest overdrive the distortion profile is as depicted. And with harder overdrive the magnitude of distortion increases (this including higher order harmonics), second and third harmonics becoming less prominent overall.

With a push-pull circuit the same triode produces almost no even order harmonic distortion at all, as usual.
This is a harmonic profile pics of a triode single-ended.
I'm not aware of any solid state component that can do this across wide range of output levels.
The circuit is a 6sn7 / ne5532 hybrid.

The Newcomer's Forum / Re: I want to ask a question
« Last post by Jmatt on October 11, 2019, 10:47:17 PM »
Thanx Phil for the reply
The chasis is a gs 65D and there is a fuse that is blown inside. The fuse is 3.5A.
The Newcomer's Forum / Re: I want to ask a question
« Last post by phatt on October 11, 2019, 07:23:27 PM »
There are likely quite a few small amps like yours that don't have short circuit protection on the output, so likely you have blown the output transistors.
Pull the chassis and take some pictures of the internals and some here might know how to fix the issue.
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