2 Unrelated questions -buffers and transformer mounting

[Adam M.]

These questions may have been better split, but since they are both simple questions I figure I'll ask them both in the same thread.

Is it wise/typical to run an input buffer with a gain higher than 1 to boost the line level input before it hits the power amp stage?

When mounting a toroidial transformer, is it normal to have the mounting bolt on the bottom(outside) of the chassis? Avel documentation also says the mounting bolt must be insulated from the case, how is that normally done?

Guess that was 3 questions, thanks for any help though.

-Adam

[ble0t]

I wouldn't see a problem with higher gain...

As for the transformer, I think what they mean is to avoid a shorted turn...see Brian's buidling guide (page 21) for a better description. It can be found here.

Hope that helps.

[Nuuk]

If you are worried about the transformer bolt issue, use a nylon nut and bolt or cable ties instead of the metal bolt.

But if used properly, there is no problem with the metal bolt and it can be used in either direction.

[Joe Rasmussen]

Originally posted by Adam M.

Is it wise/typical to run an input buffer with a gain higher than 1 to boost the line level input before it hits the power amp stage?

If you need more gain, that can be had from the gianclone chip - indeed I run mine at 30dB plus, mainly because my SACD/CD Player is 11dB down from the usual standard 2V RMS (I need the extra gain). In fact I believe it sounds better that way, even if using normal CD Players. More gain = less feedback. Actually measures higher THD, typically 0.1% vs 0.03%, but that means nothing as it is often the case that amps which relies on feedback for low THD often sounds worse than those that don't rely on it, even if they measure worse. I call it "Hiraga's Law."

So in conclusion, there is really not much need for gain in the buffer. BTW, the conventional term 'Buffer" usually describes a Unity Gain stage.

Joe R.

[carlosfm]

I also use an input op-amp as a buffer (as you may have noticed ) and prefer to put the appropriate gain on the power op-amp chip.
For a power amp, 22x gain.
For an integrated, 30~33x gain.

[Nuuk]

A timely reminder (I hope) that you can have too much gain! (This is an all too common problem with many systems assembled from commercial components)

If you want to use a stepped attenuator rather than a pot, and you want an affordable 12 step version, the more gain that you have, the larger your attenuation steps will be!

[Adam M.]

Thanks for the help everyone, and Nuuk your reminder was in time, I haven't started construction yet, and have a ton of cheap resistors I can experiment with. What would you say is too much gain?

[Nuuk]

What would you say is too much gain?

That's a 'how long is a piece of string' type of question Adam.

It depends on how many sources you use and the range of their output levels. Also, on the efficiency of your speakers.

Basically, I would say don't go above a gain of 30 for your GC and if you use a preamp/buffer make it unity gain.

If you want to use a 12 step attenuator (which I recommend over any pot), it is really a question of a bit of experimenting to find the range of attenuation that works best in your system; ie it gives you enough attenuation without each step being too big.

[Joe Rasmussen]

Originally posted by Nuuk
If you want to use a stepped attenuator rather than a pot...

You guys are yet to taste the delights of a "silver-carbon" pot, the pot you must have if you must use a pot!!!

I sourced some from an obscure source (not audiophile) and use them in my JLTi.

I had an interesting experience, this guy phoned me from Germany and he said that it intrigued him that the website said I used a "silver-carbon" pot. We ended up chatting for an hour (he was paying - not sure what Germany to Australia call costs are). He said he had come across an 'audiophile' pot on a German website and in his translation were called "silver-coal" - getit? Coal is the same as carbon - in german. I looked and found it on the site and it was a stereo pot but did not look like the one I used. Can anyone of you guys help me, I recall this company which sold audio parts and modules, including switched attenuators. Thiele or Thiel?

To continue: My german friend called them and asked about this strange pot and got an admission:

At a fraction of the price it wasn't far behind their switched brothers in sonic excellence. Hey, that's what my silver-carbon pots sounds like too.

These pots, unlike 'conductive plastics' which are all colured, are so clean and transparent, they don't sound like a pot.

I did this A-B test on a local friend, he was coming over to pick up some gear I'd been working on. I told him on the phone he was going to be my guinea pig. Between my SACD player and my RTP preamp (uses DACT) I inserted a pair of resistors forming a voltage devider, approx -10dB. I also inserted the s-c pot and a high quality gold plated toggle was able to switch between the two. The pot was adjusted so the output terminals I had the exact same levels with 0.05dB of each other.

Now Nick, do you follow me? I am comparing a pot against a refence pair of decent high quality resistors, not gainst another pot. This is near the ultimate test for a pot.

My friend Les arrived. Sat down in the hotspot seat. I told him I would play A first as a reference, and then when he was ready for it I would switch to B and after a while, would like to see if B showed any deterioration against A.

Les didn't have a clue what I was doing, didn't know this was a pot test.

Les, after about 40 minutes, switching back and forward, sometimes even when the track was playing: "B is ever so slightly soft against A and also slightly less focus." I asked if there were any colourations induced such as midbass plumminess of mid or treble changing any tonal character? "Absolutely none!"

Try this test with any conductive plastics and the colourations stand out like a sore thumb! I mean Alps Black or Blue, or the Noble pot, any conductive plastic pot.

I have tried to find this German website again and no luck. It is called Thiel or Thiele (not the loudspeaker) and I think it was entirely in german. I tried Googling it, but no luck. Too many with that name or similar.

Joe R.

[carlosfm]

A good and decently priced pot would attract me much more than a stepped attenuator.
Now that you talk about that pot, I'm really interested.
Gonna google a little to see what I can find...
Oh, now I remember... I don't speak german.