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SE 6L6 power amp, 12AT7 driver (design by Suncalc)
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Author:  mwhouston [ 14 Mar 2017, 04:52 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Geek wrote:
Suncalc wrote:
If the amp sounds good to your ears, I wouldn't even think about the CRO data. Sometimes I intentionally don't measure frequency response just so that the raw data doesn't distract me from how the amp really sounds.

That's a key reason a lot of ultra-high-end boutique manufacturers don't even spec their amps... your ears like it or your ears don't ;)
(I literally tell my customers, "Want great specs? Go to the Best Buy techs. Want great sound? Stick around!" :D )

But if bass response is an issue, I'll toss out the CFB trick of running the 6L6 cathode throug the speaker side of the OPT - it desaturates the transformer somewhat and lowers the tube anode-Z, getting you more bottom with any existing transformer... some more than others :beerchug:


Will you provide a schematic of this? I can't visualise it.

Author:  Geek [ 14 Mar 2017, 05:14 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Here ya go!

Transformer phase is important... the right way = music. The wrong way = oscillator ;)

Don't worry about any DC on the speaker... the low DC resistance of the secondary makes this moot.

Author:  mwhouston [ 14 Mar 2017, 05:23 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Ta. Never seen it before.

Author:  mwhouston [ 14 Mar 2017, 05:34 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Do I retain the same Rk. Or does it change due to the resistance of the secondary?

Author:  Geek [ 14 Mar 2017, 05:59 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Resistance in the secondary at most will be an ohm? Probably 300mOhms by the little trannies I have laying about.

Never seen it before.

Because so few of us use it. A few commercial builds do, but it's practically overlooked by the DIY community.

Author:  mwhouston [ 14 Mar 2017, 06:17 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Would GNF improve bass. And what would I lose?

Author:  Geek [ 14 Mar 2017, 13:57 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Yes, it would improve bass, but you lose gain and some dynamics (not as fast). Rule of thumb nothing too much is lost with 3dB gNFB for triode, 5dB or so gNFB for pentode. YMMV

Author:  Suncalc [ 16 Mar 2017, 10:28 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

mwhouston wrote:
if I upgraded to the 15W Edcor OPTs (15-8-5K) would I gain more bass?
Simple answer is 'yes'. The larger 15W transformers have a larger primary impedance which will yield a lower frequency rolloff. It will also likely have somewhat lower distortion at low frequencies (even though the human ear can't really detect it). It will also reduce the transient response. It's a trade off.
Geek wrote:
I'll toss out the CFB trick of running the 6L6 cathode throug the speaker side of the OPT
So lets talk feedback for a minute. There are lots of ways to apply feedback and all feedback performs the same function. Any differences in feedback response are usually due to the different impedance functions applied to the feed back loop. Contrary to popular opinion, there is no magic in any feedback function. The effects of feedback are to reduce gain, increase bandwidth (this is actually a function of reduced gain with the same rolloff points, but the effect is real), reduce circuit induced distortion and noise, and decrease gain function sensitivity to circuit parameter variation. And it works. We've been using feedback amplifiers widely since Harold Black introduced the concept back in 1934.

Now let's discuss the real issue. Feedback fundamentally changes the character if the amplifier. By it's very nature (and this is demonstrated by both testing and the mathematics), the ability of feedback to reduce harmonic distortion is directly proportional to the order of the distortion. This means that the largest effects are on the fundamental waveform. This is what accounts for the reduction in overall gain. The next highest effect in on the second order harmonics, then third, and so on. So what this means is 1) the resultant THD will be lower (usually significantly lower) with feedback than without and 2) the fundamental harmonic structure with feedback will be shifted such that higher order distortion has far less reduction that lower order distortion. And this is where the disagreements over feedback begin to arise. Because of the way the cochlea works and the way brain interprets sound, small magnitude higher order harmonics have a significant impact on the way the overall tone sounds to the ear. As such, due to the differences in harmonic structure, feedback fundamentally changes the sound of an amplifier. This is all fact; plain, provable, tested, and widely accepted.

But here's the problem. Everyone's brain works differently. And everyone hears sound differently. What's more, the brain is highly plastic in operation. As you listen to an amplifier, it literally reprograms your brain as you listen. This is one of the main reasons you can go back to an amplifier you haven't used in a while and it "sounds" different than you "remember". Usually it's not because the amplifier has changed, it's because your brain has. This is a fundamental concept that is lost on many people.

And this explains why it is so easy for some people to get sucked down the "this amp sounds better than that amp" rabbit hole. Everyone hears the same sound differently. What one person finds pleasant, another may not.

As an example, I personally am not a fan of feedback in audio amplifiers. I simply don't care for the effects it has on an amplifier's sound to my ear. But as it turns out, I've spent many years listening to simple, zero feedback, SE amplifiers with relatively high levels of 2nd harmonic distortion and very low higher order distortion. And it was precisely this type of amplifier on which I listened to music in my formative years. This has likely reprogrammed my brain to such an extent that I may never really care for feedback amplifiers. But someone that has spent their whole life listening to high open loop gain, high feedback amplifiers, as most commercial audio amplifiers have been designed for the last 40 years, will likely find my amplifiers unkind to their ears. This is NOT to say that my preferred amplifier sounds objectively better than another's amplifier; or their's better than mine. Objective assessments are determined with test equipment, impressions are formed with the ear and the brain. And no two are the same.

This is the reason one should never be too quick to pass judgement on the quality of sound out of an amplifier. It's easy to tell what you like and what you don't. It's much, MUCH more difficult to form an objective assessment of the sound of an amplifier. Simply because that is not the way the human brain is wired.

As to Geek's suggestion, I would recommend that you give it a try. You may find that you really like how it makes the amplifier sound or you may not. But the only one that can determine that is you; because only you have access to your ears and your brain.

Author:  mwhouston [ 16 Mar 2017, 16:53 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

As usual there is no free lunch. I may just stay with what I have. I'm not really and A/B man or an experimenter. I find a cct. I like and build it. I only change things if I feel there is something wrong.

One thing this amp does have is good speed, a bit of punch and drive. For the sake of a few extra lower hZ I keep it as is.

Author:  vvelt [ 20 Mar 2017, 06:36 ]
Post subject:  Re: SE 6L6 power amp, 12AT7 driver (design by Suncalc)

Interesting discussion on negative feedback, which I'm on the fence about. 

I've tried cathode feedback, using the bypass cap alone, as well as using both the bias resistor and bypass cap (Geek's diagram). I've also tried global loop feedback to reduce noise and improve bass. But to my ears these techniques rob some of the highs while boosting the lower-mids. Everything sounds fatter, but the sparkle and dynamics are lost and I don't feel like I'm getting any real bass extension.

So is cathode feedback better than loop and how so? Can CFB be adjusted, or is it all or nothing?

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