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Old 29th May 2003, 03:51 PM   #1
diyAudio Member
Join Date: Mar 2003
Location: Zürich
Default Problems with OTP's

Hi folks

A time ago I started with DIY. Build an OPAMP (prohibited word in this forum) based headphone amplifier, couldn't resist to build a OPA549 based gainclone... My first tube project is the Morgan Jones headphone amplifier to use with either Senn 580 and 590. Still a hum problem (building is fine, but solving problems is boring and somewhat depressing)

Last week I started with a SET based on the schematics posted by planet10. It uses 1/2 ECC83, EL84 and originally a EZ81. I'm using a SS retification. When the amplifier is fine I'll build a PSU with the EZ81.

I build one channel and used a OTP with 5k (primary) - 5R (secondary). I'm listining to it now, through an "audiophile" Interaudio speaker. What can I say? works.

I have some other OTP's laying around. With a multimeter on resistance position I seeked for the "speaker side" and for the "amplifier side". Then I put 2V at the speaker side and measured around 70 on the other side. This must be a good trafo to use in my amplifier, I thought.

I connected this OTP instead of the first mentioned, in the circuit and connected the multimeter to the output (speaker terminals). I turned the whole thing on and after 10 seconds the multimeter started to measuse 5V AC at the output. The speaker revelated this 5V where not music...just the weirdest scratch. Reconnected the first OTP and those 5V where there again. I saw I inverted the speaker connection side, and although I thought that wouldn't make much difference, changed them again and now it's playing again. So inverting output makes difference.

I turned back to the other OTP and reconnected it, with the speaker side inverted. Well, it sound better, but still a lot of scratch (no more 5 V).

I know I can make a lot of tests to find this out, but decided to post the whole problem here. Probably you know what the problem can be.

Some more info about the problematic OTP. It is from an old stereo tuner and gramophone from Philips, model FX 652. It has 8 connection points. I identified four of these. I think one of the four left is for a 800ohm speaker. But what about the other three?

Thanks in advance

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Old 29th May 2003, 07:42 PM   #2
diyAudio Member
Join Date: Feb 2003
Location: Birmingham, UK

Does your SE amp have a negative feedback link from the secondary of the output transformer to somewhere else in the amp (probably to the cathode of the one half of the ECC83)?

If so, the most likely cause of the 'scratching' was that the link was accidentally connected the wrong way around when you changed the transformer, giving positive as opposed to negative feedback, thus causing instability and oscillation.

The transformer from the Philips FX652 should work, as this set used EL84s in the outputs, being driven by the triode section of an EABC80 for one channel, and an EBC41 for the other channel. I have not seen the schematic, but if it followed popular Philips practice then the transformer might have a 'hum bucking' coil, used in the HT smoothing circuit, which could account for perhaps two of those connections. It is unlikely that there would be a connection for a 800 ohm speaker, as this was not standard practice in 1956 radiograms.
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Old 29th May 2003, 08:59 PM   #3
diyAudio Member
Join Date: Mar 2003
Location: Zürich
Thanks for the reply.

It really has a negative feedback loop. I will pay more attention to the connection when I build the other channel. Or maybe I'll build another amplifier, with the same tubes, but without feedback. Just like the schematic posted by Fragman56.
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Old 30th May 2003, 07:34 PM   #4
diyAudio Member
Join Date: Feb 2003
Location: Birmingham, UK
Your old Philips transformer might have a seperate winding for a feedback loop - your eight connections could be two for the speaker, two for the NFB, two for output anode and HT feed and two for the hum bucking coil.

Although I think it is unlikely that there are connections for a 800 ohm speaker I would not discount it entirely - around the time your radiogram was built Philips manufactured 800 ohm high quality speaker units for use with novel tube transformerless output Hifi amps they were making at that time. Since they were specifically designed for use with these amps I doubt that any other set had connections for them.
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