Cornet Heater Diodes?

andrew · « **on:** 31 Jul 2005, 08:33 pm »

Hi Jim

I tried some Stealth diodes in my Cornet as an experiment. I'd been suffering from noisy diodes and was trying to cure the problem. Perhaps I had a couple of bad batches of 1N5821s, I tried two separate sets, and I was never really comfortable with the noise, which on my system was making its way into the audio chain.

I saw exactly what you predicted in another thread; heater voltages too low, but it did solve my hum problem, with diodes in place the circuit is quieter and the background more inky black but the sound is poor due to the low heaters. Unfortunately, to get the heater voltages back up I need to reduce the R on the heater filter, or increase the C. If the R is dopped the subsequent reduction in smoothing means the hum just gets through again. So, this is not really a solution.

I've been searching for replacements, perhaps you might have an alternative for anyone who find the 1N5821 too noisy in their system.

many thanks,

hagtech · « **Reply #1 on:** 6 Aug 2005, 10:57 pm »

Changing the C won't help as ripple is not really the issue. It's the voltage drop across the R. Hey, I too would like to hear if anyone has found better schottky diodes.

(Note, I didn't use schottky's in the Trumpet)

jh

andrew · « **Reply #2 on:** 8 Aug 2005, 11:48 am »

Quote from: hagtech

Hey, I too would like to hear if anyone has found better schottky diodes.

jh

Hi Jim

How about the FES8DT or equivalent? From either

http://www.st.com/stonline/books/pdf/docs/2967.pdf

http://www.vishay.com/docs/88600/fes8jt.pdf

http://www.rectron.com/data_sheets/sf81-sf87.pdf

Forward voltage drop is a bit higher, meaning you would need a slightly lower R but ripple would not be significantly worse. This is just about the highest efficiency super/hyper soft diode I can find.

Do you think these are close enough? I would suggest a new R of 1.3 would you agree.

cheers,

hagtech · « **Reply #3 on:** 8 Aug 2005, 11:23 pm »

Looks to me like voltage drop is way higher. Almost double. And remember, there are two diode drops in the circuit to compensate for. I'd estimate an extra loss of about a volt.

jh

andrew · « **Reply #4 on:** 9 Aug 2005, 12:26 pm »

Hi Jim,

Finding soft recovery types with a very low voltage drop is a challenge. How about the STTH302? Not quite a low Vf as the 1N5821 but as about close as I can find.

http://www.st.com/stonline/books/pdf/docs/8319.pdf

thanks,

hagtech · « **Reply #5 on:** 9 Aug 2005, 11:29 pm »

Quote

Finding soft recovery types with a very low voltage drop is a challenge

I don't think you'll find one.

jh

andrew · « **Reply #6 on:** 24 Aug 2005, 12:03 pm »

Hi Jim,

I tried these last night. I had to drop R21 to 1R by placing two 2Rs in parallel, I will tweak this value when I have more time. I expect about 1R2 would be best.

The Stealth Diodes were much better than the original 1N5821s but the heaters were too low.

First impressions with the STTH302s are: it is running at least as quite as it was with the Fairchild Stealth Diodes but the heaters are now obviosuly running at a better voltage.

In comparison with the 1N5821s, there is definitely less hum even though the heaters now have more ripple on them. I can only guess that in some systems the diode switching noise is more significant than the added heater ripple caused by the reduction in R21.

If anyone is struggling with a noisy/humming Cornet and has been through all of usual checks then this might be worth a try.

cheers,

bluesky · « **Reply #7 on:** 3 Sep 2005, 03:40 am »

Hi Everyone

PCX is now stocking Cree Shottky diodes and I am wondring if these are suitable to replace the 1N5821s. If they are suitable, which rating or value is the most suitable?

Cheers

Bluesky

andrew · « **Reply #8 on:** 5 Sep 2005, 07:45 pm »

Do they have the datasheet, the 1N5821 are ideal voltage-wise as they have a very low Vrf....but in my system with a reasonably sensitive power amp and speakes you can hear the switching noise....

bluesky · « **Reply #9 on:** 15 Sep 2005, 07:42 pm »

Hi Andrew

My aplogies for the late reply, my internet has been down.

I don't know about the datasheet for the Cree diodes but there is a thread going on the Aksa forum if you want to read that one. These might be the "next generation" of diodes from my (very) limited knowledge of audio electronics, they can be quite pricey though.

Cheers

Bluesky

andrew · « **Reply #10 on:** 20 Sep 2005, 08:33 pm »

Hi Bluesky,

It certainly might be worth digging out the data sheets for these; I'll take a look as soon as I have time. At present I've got some STTH302s in and they seem to be a good compromise between noise and forward voltage drop.

cheers,

bluesky · « **Reply #11 on:** 20 Sep 2005, 09:20 pm »

Hi Andrew

Keep me posted on what happens!

Bluesky