Explendid amplifier designed by Michael Bittner, our MikeB

[MikeB]

Yahoo !

But, shouldn't groundloops create hum instead of radio reception ?
Sheldon, did you connect ground to earth ?

Mike

 

[anatech]

Hi Mike,
A ground loop can induce hum and also lot's of RF. If you have one, it's easy to have the other.

-Chris

 

[Sheldon]

Hi,

Yes Mike, the grounds are connected to earth. No real hum though.

I clearly had a grounding problem. But I'm still curious as to why I picked up radio. I'm not versed in radio technology, but I would think that one would need some kind of RF oscillating circuit to sympathetically vibrate with the radio signal and amplify the modulation as audio. Does that mean my boards are oscillating somewhere? As I noted, neither tube amp show this. BTW, it was a Christian radio station. Does God love solid state more than tubes?

Sheldon

 

[anatech]

Hi Sheldon,

BTW, it was a Christian radio station. Does God love solid state more than tubes?

You just proved he does.

When is a ground not a ground? When there is enough impedance to cause a problem. All wire has inductance and capacitance (to other conductors around). The signal was probaby rectified and amplifed as audio. much like an AM receiver.

-Chris

 

[Sheldon]

When is a ground not a ground? When there is enough impedance to cause a problem. All wire has inductance and capacitance (to other conductors around). The signal was probaby rectified and amplifed as audio. much like an AM receiver.
-Chris

So just the various wiring outside the amp, along with maybe some input components (resistor, cap) is enough to set up a sort of tuned circuit? No need to necessarily worry about the amp?

This got me curious, so I did a little testing with an AM radio (tuned to the bottom of the dial). If I pass the antenna over the new amp, I get a good buzz, but little with the two original amps. The buzz goes off the instant the switch is turned off. That suggests to my simple mind, that it's rectifier noise, as the amp is still quite alive for some seconds. In the mono's I had used some ceramic caps across the diodes, but not in the stereo amp. Maybe worth while going back and adding them.


Sheldon

 

[anatech]

Hi Sheldon,
Good plan. That does sound like RF hash from the rectifiers.

-Chris

 

[Upupa Epops]

Gentlemen, diode is using for detection of AM modulation... Not hash from rectifier, but detection of some near strong transmitter...

 

[clem_o]

If I pass the antenna over the new amp, I get a good buzz, but little with the two original amps. The buzz goes off the instant the switch is turned off. That

That certainly sounds familiar... and it is indeed a grounding problem. Are the new amps in a box? Could be missing a ground plane.

Cheers!

 

[anatech]

I was thinking bursts of HF at turn off of the diode. Depending on the capacitance for filter caps which would shorten up the conduction angle.

-Chris

 

[clem_o]

I was thinking bursts of HF at turn off of the diode. Depending on the capacitance for filter caps which would shorten up the conduction angle.

Certainly sounds like a plausible explanation. I never did figure out what caused my problems, but they did get fixed by better grounding... Putting a small capacitor on the rectifier diodes should address it?

Cheers!

Clem

 

[anatech]

Hi Clem,
It has sometimes in my experience.

-Chris

 

[Sheldon]

That certainly sounds familiar... and it is indeed a grounding problem. Are the new amps in a box? Could be missing a ground plane.

Cheers!

They are in the box shown a page or two back. I was, as usual, in a hurry to try them out. I think the grounds are ok, but I'll double check (but maybe not before I head out for a couple of weeks). FWIW, it definately sounds like 120Hz, and one side of the stereo pair sounds much louder than the other. Maybe a weak (leaky) bridge? They are running close to spec. or maybe a bit over (PIV)( hurry up mode again). I planned to replace them with better units anyway. We'll see how that works.

Sheldon

 

[MikeB]

Hi Sheldon, aren't you using metal bridge rectifiers ? I thought these have typically very high ratings like 25A250V ?
BTW, you don't need much electronics for radio reception, all you need is an antenna, some diode device and a headphone. Look at these radios done by soldiers in 2nd WW, they used a headphone, a needle, a rusty razor blade and an antenna...
I don't think that your symasyms are oscillating, they would sound very bad if they do. You could try a RC on the AC-side of the rectifiers, like 100nF+500ohms.
You're right, an AM-radio shouldn't show any symptoms when held above the amps...

Mike

 

[Sheldon]

Hi Sheldon, aren't you using metal bridge rectifiers ? I thought these have typically very high ratings like 25A250V ?
BTW, you don't need much electronics for radio reception, all you need is an antenna, some diode device and a headphone. Look at these radios done by soldiers in 2nd WW, they used a headphone, a needle, a rusty razor blade and an antenna...
I don't think that your symasyms are oscillating, they would sound very bad if they do. You could try a RC on the AC-side of the rectifiers, like 100nF+500ohms.
You're right, an AM-radio shouldn't show any symptoms when held above the amps...

Mike

These were some I got at surplus and may be rated as low as 50V, so in hindsite it may be no surprise that they are gagging. I'll switch them out before one dies and takes something with it. BTW, it seems that one is quiet and the other noisy, maybe another sign of living at the edge (or over the edge in this case).

I remember the old crystal radios we made in Boy Scouts. Hadn't thought of that, but yes, just an antenna, a diode, and a coil of wire for a tuner.

Sheldon

 

[PMA]

I remember the old crystal radios we made in Boy Scouts. Hadn't thought of that, but yes, just an antenna, a diode, and a coil of wire for a tuner.

Sheldon

Put symasym behind this receiver and go ahead!

It usually worked only with antenna, diode and high impedance headphones ... . Simple is the best, as hifidiyguys used to say ..

 

[MikeB]

These were some I got at surplus and may be rated as low as 50V ...

That's a big ouch ! Assuming you use a 36v supply, the diodes have to withstand >70v... I don't know if this fact is accounted for in the rating for rectifiers. Anyway, you really should get some nice metal bridges with ~200v25a rating, these are not expensive and very tough. Or some MUR820/860 diodes...

I use KBPC2502, rated 200V (max reverse peak),25A max average current and surge current of max 350A.

Mike

 

[op220fz]

HF

My sysasym 5.2 run in a wood box without HF injection.

And i have some HF TRANSEVERS and serverals Antenne only ~5 meter away.

A BJT can decode HF to AM - B-E diode !

I think on the 300pF ...

look: RD Sniffer
- http://www.g3ycc.karoo.net/sniffer.htm

 

[Sheldon]

That's a big ouch ! Assuming you use a 36v supply, the diodes have to withstand >70v... I don't know if this fact is accounted for in the rating for rectifiers. Anyway, you really should get some nice metal bridges with ~200v25a rating, these are not expensive and very tough. Or some MUR820/860 diodes...

I use KBPC2502, rated 200V (max reverse peak),25A max average current and surge current of max 350A.

Mike

Right you are. That's what I used the first time (200/25A). Sometimes I'm too impatient to wait for the right parts. But everything else is fine, so proper rectifiers will go in as soon as I return from a trip.

Sheldon

 

[PMA]

It is all about shilelding and interconnets topology only, nothing else. I posted here the flawless diagram several months ago.

 

[Sheldon]

It is all about shilelding and interconnets topology only, nothing else. I posted here the flawless diagram several months ago.

]

And I printed that one out and filed it, thanks BTW. Right now I'm just testing and measuring. When I get all the amps built and the speakers measured and crossovers sorted (whether digital, or line level active or passive), and I'm ready for final placement, I'll drag out that reference and work on proper interconnects.

Sheldon