Hi Carlos,
In a IGC configuration the chip will see the input resistor plus the output impedance of the previous stage. So the impedance that the chip see's can't go down much. Maybe 10K to say 47K or so, based on what I see in other posts.
Does this affect the sound ? Any explanation for this ? Or is the chip succeptible to oscillation with high Zin.
Thanks,
Ashok.
Terry_Demol said:
Yes, and others too. It does seem that bipolar inputs need proper balancing, and not just DC. Removing by-pass cap in earlier JLTi config confirmed that and I now agree what I thought was an assertion of yours earlier on. But when you combine that with Lower Z T-network, another level again is reached.
Agree, but interested if you come to same conclusion independently..
CLIO
At 0dBu, putting Clio into a loop, the THD is not all that good @ 0.097% - 2nd harmonic hugely dominent. But for some reason, at -10 dBu and increasing sample size and adjust sample freq, it is possible to get as low as 0.017% with a noise floor -120dB, again relative to 0dBu.
Joe R.
ashok said:
Hi Ashok
Remember, on an IGC the input resistor is actually part of the feedback loop, and also sets the gain of the amp.
If the previous stage is active it will have constant (and also ideally low) output impedace, much better than a pot.
If that previous active stage is on a separated box some distance away (like a standalone pre) the feedback loop will be huge, and the amp may even pick up noise from the interconnect (acting as an antenna).
The IGC works better with an input buffer, even on a power amp being driven by a standalone pre.
It works without an input buffer, but there are some issues.
With an input buffer it also sounds better.
It may seam strange for some.
But everytime I use a good active stage between a pot and a power amp I have big improvements.
Ultra-minimalism is only good in theory.😉
Hi Carlos,
Thanks for your reply.
If the previous stage is reduced to a voltage source and output impedance (or R source) , the next stage (inverting) will add this Zo ( or Rs) to the Rin of the Inverting stage and treat it as a larger resistor . While it does affect the gain , it only provides an input current ( theoretically) to the Inv stage , independant of the following stage.
The -ve input is at a virtual ground ( 0 Volts).
So actually the chip is now receiving an input current that depends only on the series impedance between the -ve input and the voltage source.
It's possible that the chip also has poor common mode performance which is why it sounds better in inverting mode.
Wonder if anyone has checked the spectrum in both modes with identical gain.
I have also found a good buffer between the power stage and signal source helps. It generally seems to sound better to the ear.
Cheers,
Ashok.
Thanks for your reply.
If the previous stage is reduced to a voltage source and output impedance (or R source) , the next stage (inverting) will add this Zo ( or Rs) to the Rin of the Inverting stage and treat it as a larger resistor . While it does affect the gain , it only provides an input current ( theoretically) to the Inv stage , independant of the following stage.
The -ve input is at a virtual ground ( 0 Volts).
So actually the chip is now receiving an input current that depends only on the series impedance between the -ve input and the voltage source.
It's possible that the chip also has poor common mode performance which is why it sounds better in inverting mode.
Wonder if anyone has checked the spectrum in both modes with identical gain.
I have also found a good buffer between the power stage and signal source helps. It generally seems to sound better to the ear.
Cheers,
Ashok.
This is distortion of 4780 at 32 W / 8 Ohm, 6 kHz, noninverting mode ( spikes at 15625 Hz is relict of TV, at 19 kHz is pilot signal of FM tuner ) - resistors are normal 0207, any Caddocks 😉 .
Konnichiwa,
Actually, it works absolutely stonking in reality too. But you got to do it right...
Sayonara
carlosfm said:
Actually, it works absolutely stonking in reality too. But you got to do it right...
Sayonara
Kuei Yang Wang said:
Hi Kuei,
As a "wire with gain" is still impossible, please tell me how to avoid an active stage (comparing to a passive pre with a pot ) and have similar results soundwise.
My post was related to this matter, it was not a generic assumption.
Konnichiwa,
I find it eminently possible. Simply wind the wire around a Mu-Metal Core.
I can't tell how to have similar results (nor would want such), but I could tell you how to get better ones...
Sayonara
carlosfm said:
I find it eminently possible. Simply wind the wire around a Mu-Metal Core.
carlosfm said:
I can't tell how to have similar results (nor would want such), but I could tell you how to get better ones...
Sayonara
Kuei Yang Wang said:
😀
Kuei Yang Wang said:
Tell me.
I will ditch my new AD815 preamp.
Remember, I'm talking about pots, not expensive signal trafos.
Just a follow on from Joes Reply to myself. With the 10K resistor to non inverting input the DC offset is down to about 20mV which is more than acceptable for most peoples needs.
Don't get stressed, and go do it - this mod rocks !!
Shoog
Don't get stressed, and go do it - this mod rocks !!
Shoog
I thought that the main objective here was to not use a resistor from NI to groud?
Of course, that's the obvious way to reduce DC.
Of course, that's the obvious way to reduce DC.
I converted my amp from 47k / 18K/150R/18K and 13K (with trim evaluated) from NI to ground.
Before, I did not post the measurement, because the amp showed some hum. The reason was a broken joint at the ground rail (caused from my modifications), as I found out later.
Now I use like Joe 22K / 10K/100R/10K and 10K from NI to ground. The bias at output is now on one channel -18mV on the other -38mV.
What are cryo-ed BBs?
Sometimes it is very hard for me to follow the postings from Joe, because all this abreviations...
Franz
Before, I did not post the measurement, because the amp showed some hum. The reason was a broken joint at the ground rail (caused from my modifications), as I found out later.
Now I use like Joe 22K / 10K/100R/10K and 10K from NI to ground. The bias at output is now on one channel -18mV on the other -38mV.
What are cryo-ed BBs?
Sometimes it is very hard for me to follow the postings from Joe, because all this abreviations...
Franz
Franz G said:
18mV & 38mV - excellent.
So far all have been sub 50mV with those values, hopefully that trend will continue. Actually the have been sub 40mV - but sub 50mV is a reasonable target.
Sorry, I knew that Terry would know. But these have been cryogenically treated. Believe me, this is something we all need to get onto. The difference before and after treatment almost defies explanation. Once heard, you never go back.
Terry and I have a friend who is setting up a cryo service, the idea is that you can send some off your favourite 6DJ8/ECC88 (or whatever) with a fee and they get returned cryo-ed. First he will put it on a tube tester to see if the condition is OK. It will take some time before the service come online, but it's very exciting. I will be assisting in any way I can and when ready - I will certainly announce it.
Joe R.
carlosfm said:
It's not only to reduce DC, but also to balance the input with respect to (WRT) AC - or signal. So if the (-) input sees 10K to ground, so should (+) - and don't trim to reduce offset. Better to find a set of values that will give you sub 50mV DC offset, and thus we're hoping that 22K/10K/100R/10K & 10K on (+) will achieve that. So far so good. Franz has also tried that combination and has reported 18mV and 38mV (L & R) - so that makes 6 amps (12 channels/chips) that has achieved the target, hope the trend continues.
Joe R.
Joe Rasmussen said:
Agreed on the Cryo Treatment. I send mine to Calgary to get done... the best description of what he does and the (claimed) benefits -- there certainly are benefits -- are best described in one of his eBay auctions.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=67816&item=5720410136
This is the same guy who makes the PEARL tube coolers.
Bill can do your tubes now -- just remember he is 1 guy and like most 1 guy operations is often behind schedule (that would descibe me too 🙂) I've known Bill since 1972 when i visited his shop to see Radford S90 boxes being made.
dave
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