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Old 8th September 2012, 10:19 PM   #2011
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Hi Ale,

Quote:
Originally Posted by mogliaa View Post
I did not follow your advise in the end after some tests ended up wiring up it on 5.6:1 so sacrificed output impedance a bit.
Actually 5.6:1 is how I would use it, that gives the lowest possible output impedance, perfect to drive difficult loads like TVCs or long interconnects to the power amp

Best regards

Thomas
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Old 8th September 2012, 10:24 PM   #2012
mogliaa is offline mogliaa  United Kingdom
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Ops, sorry! I know, I wired it in 5.6:2 to get a tad more gain as I needed...
Don't have TVC and I think output impedance is around 1k with this setup...
Ale
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Old 9th September 2012, 10:15 AM   #2013
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Hi Ale,

if you don't have to drive a TVC that's ok. The 1k output impedance are much better than the 7k many 26 preamps have.

Best regards

Thomas
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Old 11th September 2012, 10:01 AM   #2014
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There's quite a lot of talk about not mixing signal ground and filament supply ground, but I guess that is for cathode/battery bias designs? For filament bias should I only ground at the reg filament negative?

Making baby steps with this preamp, but it is coming together. I let the smoke out of a few things yesterday so need to replace those and should be good to continue!

Slowly slowly...
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Old 11th September 2012, 10:08 AM   #2015
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Charlie, just don't connect the Filament Raw dc supply to Ground. to be certain: don't connect the transformer secondary, or the rectifier, or caps or any part of the input side of the filament regulator to ground.

Naturally, the filament bias resistor connects to ground. One side of the filament connects directly to ground if you use battery bias or fixed bias - but all of those things are on the output of the regulator.

It's just the input side that must strictly float.
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Old 11th September 2012, 10:14 AM   #2016
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Thanks Rod, yeah I assumed that it must have a ground reference somewhere - I know you guys go on about the magic of DHTs but they can't be that magic!

Think I must of had a bad joint somewhere before or something, as everything seems to be working fine now.

Thanks for your help.
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Old 11th September 2012, 11:08 AM   #2017
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Good to see you getting closer! Are you moving towards getting sound out yet?

Andy
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Old 11th September 2012, 11:11 AM   #2018
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Example (my #26 preamp wiring).
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File Type: jpg #26 preamp wiring.jpg (57.5 KB, 482 views)
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Old 11th September 2012, 12:10 PM   #2019
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Quote:
Originally Posted by andyjevans View Post
Good to see you getting closer! Are you moving towards getting sound out yet?

Andy
It's painfully slow - trying to do it in between breaks and other jobs. Going to get it performing the best I can before taking home to the system, but yes, actual sound is on the horizon!

Charlie
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Old 12th September 2012, 05:53 PM   #2020
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Default Few results

Got a few results from an initial test.

Firstly THD was quite high - 0.1%, but that settled after a while to 0.07% - seems reasonable.

Noise is quite bad, but that is expected as I still have to work on the PS quite a bit and also the PS is far too close to the preamp. It is currently at about -75dB so room for improvement.

It is however very unstable at low frequencies (<100Hz), this could be down to my PS? I have some SSHV2 boards ready to put in when everything is built - so am hoping this will help. Any ideas why else it may be so unstable?

Also, above 60KHz the signal goes through the roof - havn't seen anything like that before so unsure what the cause is.

10R 50W filament resistor got to about 110 degrees C, which seemed quite high, in fact I think it was burning up a little. So I could put 4 together to make 10R if it becomes an issue.

What is the current trend for operating points? People seem to be very happy with starved filaments...?

Getting there....

Charlie

ps these things are SO microphonic. You guys are nuts!
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