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Old 6th April 2003, 09:17 PM   #21
phonon is offline phonon  United States
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I temporarily added a cathode resistor on one of the cathodes (I only have one resistor of suitable value on hand) as you suggested. What is the purpose of cathode bias? As for lowering the input resistance, I assume this is just to lower noise pickup?
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Old 6th April 2003, 09:37 PM   #22
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Default Grid bias vs cathode bias

Using grid bias allows higher amplication which is useful for crystal mikes, but gain is not such a great priority in your application. The downside is that the tube does not amplify in a linear fashion, so frequency response is reduced - again, not a problem with xtal mikes which have a limited frequency response, or AM tube radios where upper frequency response is severely limited. Cathode bias allows a tube to amplify in a more linear fashion.

Hifi amps which use grid bias (like the Mullard 3-3) use large amounts of NFB to overcome the non-linear amplification of the first stage.

The lower value grid resistor allows your MM cartridge which has relatively low impedence to feed into a low input.

With your amp, I doubt that the values of the cathode resistors are too critical, especially as gain is not a problem and your source is not going to overload the input - you could find that any value between 1.5k and 3.3k would work reasonably well.
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Old 6th April 2003, 09:42 PM   #23
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Default BIAS.

Hi,

Quote:
The lower value grid resistor allows your MM cartridge which has relatively low impedence to feed into a low input.
Indeed, but doesn't the MM cartridge standard expect an imput impedance of 47K plus say 100 to 150pF in // to correctly load the cartridge?

What about RIAA correction?

From what I read so far there isn't any or did I miss that somehow?

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Old 6th April 2003, 09:54 PM   #24
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Default Re: BIAS.

Quote:
Originally posted by fdegrove

What about RIAA correction?

From what I read so far there isn't any or did I miss that somehow?

I was hoping that EC8010 would come back with the circuit for that! (it would avoid one of my 'suck it and see' solutions!)
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Old 6th April 2003, 10:37 PM   #25
EC8010 is offline EC8010  United Kingdom
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Default I've been busy destroying rectifiers

As previously mentioned, a MM catridge expects to see a 47k load. There is no possibility of using the existing circuits to achieve RIAA correction and gain. However, there is no reason why the existing valve bases, and possibly, valves, should not be used.

There's lots of confusion about RIAA. Records are cut very nearly constant amplitude, but with a step in response between 500Hz and 2.1kHz. Crystal cartridges (which were amplitude sensitive) could thus play RIAA records, but suffered from the step in response. Since crystal cartridges, and their associated equipment were so poor, the step was the least of their problems. MM and MC cartridges are velocity transducers, which means that their output voltage doubles for a doubling of frequency. RIAA correction (as it is understood today) primarily corrects for this characteristic, and adds an inverse step to correct for the recorded characteristic. The final characteristic required 19.1dB boost at 50Hz, and 19.6dB cut at 20kHz. The dynamic range of this equalisation makes it quite hard to achieve satisfactorily.

Older designs placed the RIAA network in the feedback loop of a valve (usually an EF86). Better designs applied the feedback over more stages (allowing more feedback). None worked very well because there wasn't enough gain for the feedback approximation to work properly at LF. (Can be done with one valve.)

Some designs used a passive network between two stages to achieve equalisation. This works, but the necessary loss of >19.1dB simultaneously causes noise and overload problems. It's really hard to design an RIAA stage like this that works well, but it can be done. (Needs two valves.)

The more modern approach is to split the equalisation into two stages, and apply the 75us HF cut first, then the 3180, 318 pairing after the second stage. Its is much easier to get around the noise/overload problems with this technique. (Needs three valves.)

It's a loaded question, I know, but which strategy do you want to adopt?
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Old 6th April 2003, 10:53 PM   #26
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Default COMBINE.

Hi,

A combination of active an passive RIAA correction is not often used but it has the advantage of:

Keeping things simple and short as far as the signal path goes.

An odd example is doing some correction between the anodes of say, one 12AX7s and some passive correction inbetween the output/input of both triodes.

It's not the easiest circuit to calculate but it has some elegance going that I find quite attractive.

RIAA isn't easy to implement correctly but when done right it beats any CD based system hands down.

Cheers,
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Old 6th April 2003, 11:26 PM   #27
phonon is offline phonon  United States
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Since records are my only material for mono recordings, I would be willing to rebuild the amplifier without a line level input; this means RIAA equalization could be done all throughout the signal chain. I am also willing to not have bass and treble adjustments. This should leave plenty of room for active and/or passive adjustements, and plenty of ability for gain; the 6SC7, 6SJ7, and 6SL7 are all twin triodes, so this is the equivalent of 6 tubes I have to work with to get from the input to the drivers.
The only tube changes I would be willing to do would be to swap some of the tubes for more 6SL7 tubes, as I have several of these from another salvaged device. I hope this helps in deciding how to implement the RIAA compensation.
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Old 6th April 2003, 11:51 PM   #28
EC8010 is offline EC8010  United Kingdom
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Default Pentode?

My RC-19 says a 6SJ7 is a pentode. But it says that the mu when triode-strapped is 19, which would be very handy. How many do you have?
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Old 7th April 2003, 12:08 AM   #29
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My mistake; 6SJ7 is a pentode. I only have one of those.
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Old 7th April 2003, 12:16 AM   #30
EC8010 is offline EC8010  United Kingdom
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That's a nuisance. I was rather hoping you might have two. As I'm sure you have realised, I would rather use the third option, but it needs two valves with mu of 10-20. You need to commit yourself here, which option do you want to go for?
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