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Old 19th March 2013, 08:30 AM   #61
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Quote:
Broskie gives an example of an IV stage using the 12AX7 w/Unbalancer
I've done some searching/reading and the 6n2p tube looks like a good candidate for the gain stage,
it has a mu of 100 like the 12ax7, but is much cheaper and excellent sounding.
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Old 19th March 2013, 01:08 PM   #62
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Quote:
Originally Posted by danny_66 View Post
Try the Rhopoint squaristor or econistor, from what i've read they are even better as i/v resistor.


+1
I have a set of econistors here in 20 ohm...will try them next...
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Old 20th March 2013, 02:13 AM   #63
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Originally Posted by lordearl View Post
Broskie gives an example of an IV stage using the 12AX7 w/Unbalancer here:

The Unbalancer

I think all you'd really need to change is the 6.81 resistor for the LM334 (not sure what value you would need). You could even try a 5751, which should sound fantastic.
The example schematic used for 12AX7 as an I/V converter shows 34ohm resistor used with the LM334 and also the anode resistors are changed from 24k to 120k.

Broskie *generally* recommends low mu tubes for the input. Isn't a 12AX7 considered a high mu? or would this be seen as an advantage in an I/V application?

I built an octal version and am currently using 6SN7s (equivalent to 6CG7) for both input and output, I/V resistors are 27ohm and I get just about enough gain into an F5 amp and I'm very happy with the sound. On days when I really want to crank it I could do with a little more gain so thinking about changing the resistor values or swapping in some 12AX7/5751 equivalents (12SL7?) in the input. Anyone have experience of 12AX7 in this position, or what are the downsides likely to be?
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Old 20th March 2013, 09:42 AM   #64
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Well, I guess typically these high gain tubes are noisier...so, tube and anode load resistor noise vs resistor noise of higher i/v and lesser distortion of the Dac chip...
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Old 20th March 2013, 11:36 AM   #65
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Thanks Blitz. So it sounds like the 6N23P is the happy medium?

Quote:
Originally Posted by Blitz View Post
...Too much fun and stuff to play with...I have on my list as well:

Good Design Bad Design and DIY: Juli 2010

(build your own I/V-Resistor with Manganin-Wire, which is 86% Copper and should sound best...)
I haven't yet tried any exotic I/V resistors. What about precision metal foils types?

This roll your own idea sounds interesting! I found a lacquered verson on-line but you have to buy 100s meters. This guy sells in small quantities and has a few different values 5Mtr. - Manganin wire =0.06mm ; 155?/m | eBay

I asked about the lacquer: 0.06 to 0.17mm is lacquered, the 0.025 to 0.05mm is fabric coated.

Last edited by Bud Barnez; 20th March 2013 at 11:51 AM.
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Old 20th March 2013, 04:36 PM   #66
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Hi Bud,
The change of the anode resistors from 24k to 120k for the 12ax7 is because the 12ax7 has a much higher internal resistance (70k).

The 12AX7 is certainly a high mu tube, but the voltage of the es9018 after i/v is very low(depends on the i/v resistor) something like 0.1v,
to get that to 2v we can use a high mu tube or change the value of the i/v resistor,
but the es9018 is much more happier with a low i/v resistor.
So that leaves us to high mu tubes.

High gain tubes are not all noisier than low gain, good examples are the 7025 and the 6n2p-ev, Here's a review of the 6n2p-ev (halfway, search for 6H2P)
Bad plate resistors can also be a source of noise.

Since you have an octal version, you could try the 6SL7/12SL7, they will give you more gain.
You'll also need to change the plate resistors to something like 82k.

To get an idea of the gain you get with different tubes, see the last pages in the aikido manual: octal, 9-pin
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Last edited by danny_66; 20th March 2013 at 04:50 PM.
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Old 20th March 2013, 05:01 PM   #67
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Mr Bud Barnez
At page 3 post 27 you can find the unbalancer schematic diagram using the ECC83 (12AX7) and ECC 82 as I/V for 4xPCM1704 you can try it with another I/V resistor. This version was simulatetd on LT Spice for the lowest THD and is runing on my system.
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Old 21st March 2013, 12:50 AM   #68
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Originally Posted by gabrielbecheanu View Post
Mr Bud Barnez
At page 3 post 27 you can find the unbalancer schematic diagram using the ECC83 (12AX7) and ECC 82 as I/V for 4xPCM1704 you can try it with another I/V resistor. This version was simulatetd on LT Spice for the lowest THD and is runing on my system.
Best regards
Thanks for your schematic. You ended up with a very similar circuit to the Broskie I/V 12AXY, other than the CCS section where you use a different arrangement with diodes.

Also I note you paralleled up 100ohm I/V resistors. Is there an advantage in that over a single resistor on each line?
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Old 21st March 2013, 01:12 AM   #69
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Danny, thanks for your reply and links. So the SL7 is the only next step up from SN7.

I am using dual mono PCM1794, data sheet says about 7.8mA so using in mono should be around 15mA and giving around 0.4V with my 27ohm I/Vs.

Like the ES9018, I read that the 1794 likes a low value of I/V too.

Your link suggest that I may be better of with a 9 pin input tube. I'd be happy to try it if it's worth it and otherwise I'd like to try some 12SL7s but, as this is my first project, I'm nervous about what other values need to be changed in the circuit. You suggest 82k plate resistors, are you able to advise what else should be adjusted to accommodate this please, or point me in the direction for how to calculate it? My B+ is 240V.

I just looked at the data sheet for 12SL7 and plate current is 2.3mA.

Any advice greatly received.
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Old 21st March 2013, 01:14 AM   #70
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I did some more testing of the tubes - my dac's output is about 120mV using 100R resistors, so theoretically the 12AX7 should get it to around the 2vrms, however before I fiddle around with the 6.8R resistor controlling the LM334, I thought I'd test it with a 12AT7, because at least the 6.8R doesn't need to be de-soldered and the mu of 60 should be good enough.

As for the sound:

GE gray play 12AT7 - very low noisefloor, nice detail in the treble, bass a little muddy/bloated

Raytheon 12AT7WC (black plate, US made) - slightly noisier, though not annoyingly loud, excellent mids, better bass than the GE

The gain levels are fine for my purposes (speakers are 90dB efficiency, power amp is 3.5v sensitivity), so I won't bother changing to the 12AX7 set up as the 12AX7 is far more expensive and the gain will probably be too much if I change to a more sensitive power amp

I've left the Tung Sol 6CG7 as the output tubes, they're neutral, sound like 6SN7 and the transconductance is reasonable esp. as I'm using very short cables anyway
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