Toob sound?

I am working on a precision tanh function generator to compress a wide range signal before digitization to increase resolution for signals near center and sacrifice resolution for large swings. It is based on the LM13700 dual transconductance amplifier. I am trying to reduce the non ideal aspects as much as possible and have come up with the following scheme:

I control the current into the Iabc pins using an LM385 1.25 volt as reference for the current regulator.

I control the chip temperature by controlling the voltage at the Iabc input pins (which runs around 1.3 volts at room temperature and is effectively two diode drops) using the 1.25 volt as setpoint and using the two darlingtons on the chip as heaters.

I drive the two sections inversely with the input voltage and subtract the output currents from each other (keeping the output swings very near center of the supply, to reduce Early effects on the current mirrors) so that any difference between the positive and negative current mirrors partially cancels for any signal polarity.

I haven't yet collected test data to see how closely I have achieved the tanh function, but my first impression of the operation makes me think of tube amplifiers. There is no sharp clipping for significant overdrives, so I suspect it would sound very much like an open loop tube amplifier. I can see lots of ways to combine two transconductance amplifiers (with a gain and full scale knob on each) to achieve a wide range of distortion shapes versus overload, so many different tube overload effects should be producible.

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its the output stage thats the issue, not the rest. IIRC THD rises to relatively high levels at 20kHz on Tr amps, so the THD specs are not comparable between low nfb valve and high nfb tr.

NT

I think you have a wrong idea about valve sound. It comes from the mechanical feedback, not from overload. And a tube amplifier limits similar to the transistor stage, do not believe that crap from the phools. And the euphonic distortion won't be simulated either, it is already there at very low levels. The 13700 is an awful piece of junk, I use the SSM2164 for VCA apps, much better noise, distortion, temp.stability etc. you can use it as a linear>log converter in an opamp feedback loop, which compensates for the temp dependency, you can beautifully implement all kind of strange transfer characteristics in the log domain.

--
ciao Ban
Bordighera, Italy

So why are there kilobuck tube line amps and preamps and mic amps?

A good solid-state amp will have *every* measurable parameter orders of magnitude better than the best tube amp.

John

Tube sound comes from self-delusion. Any amp sounds bad if it's clipped, and no amp should be run that hard. If you turn down the volume a bit, does tube sound disappear?

John

How about putting a light bulb beside a FET amplifier? After all, a tube is a thermally stabilized Field Electric Tube (F.E.T.). Oh! I forgot. A FET is a Fanastically Enhanced Tube...

I read in sci.electronics.design that snipped-for-privacy@meeow.co.uk wrote (in ) about 'Toob sound?', on Sat, 7 May 2005:

True, in some cases, 30 years ago, but not now.

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
\'What is a Moebius strip?\'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

(snip)

But the SSM2164 is a linear amplifier and I am trying to produce the hyperbolic tangent transfer function. The LM13700 performs this function, naturally. Are you saying that I can produce a more precise hyperbolic tangent transfer function with the SSM2164 (with some additional external parts)?

(snip)

Thanks, Bob, but I really don't care much about toob sound at the moment. I am a lot more interested on generating an accurate and stable hyperbolic tangent transfer function, and just got side tracked into tube musings while driving by.

>

But that is where I can hear the difference.

For me? Yes.

same reason all products exist, to make money.

NT

You are assuming that the "Tube sound" comes from soft clipping and and that it can be simply modeled by the tanh function. I submit that the "tube sound" is much more complicated than that. Among other things it involves grid rectification in class ab amplifiers and the resulting compression-expansion. Much work has gone into this and the best tube simulations use DSP to simulate the most important parameters affecting the sound. I doubt that you will get very satisfactory results with an simple analog circuit. If you want more information on this, I can put you in contact with a gentleman who has done much research in this area. If interested, e-mail me by removing the ns from the front of the address. nsmontassatyahoodotcom. Bob

Now *every* differential amplifier produces the tanh function, you do not need a transconductance amplifier for it.

• + o o | | .-. .-. | | | | ___ | |R1 | |R1 +-|___|-GND '-' '-' | | | | |\ | +-------+---|+\ | | | >-+--o Uout +-------------------+---|-/ | | | | |/ | | | | ___ | |/ \| +-|___|--+ Uin o-| |-+ R2 |>

Then maybe you can tell me why a 0.1% THD valve amp can leave a 0.003% THD tranny amp far behind in sound quality terms? The persistent mystery.

Am not convinced by 'its all in your mind,' it doesnt square with experience very well.

NT

But you can buy a solid-state amp that will be undistorted to 400 watts, so it will never be anywhere close to clipping. There aren't many 100 watt tube amps around, much less 400.

Tube sound has a few components:

1. People who think they can hear the difference, but actually can't. A lot of wine snobs are like that: they can read labels but in a blind test can't tell Gallo from LaTour.

1. People who like more harmonic distortion than already exists in the program material.

2. People who like bass boom from their speakers.

1. Guitar players who deliberately clip their amps, and like the way tube amps clip. Thst, of course, can be simulated in a ss amp.

John

I designed a transistor guitar amp once, a private-label thing for a chain of Southern US music stores. I think it was called the Ryder 400 or something silly (after Frank Ryder, one of my techs at that time and an astounding aerobatic pilot; only time I've ever been motion sick, Frank did it.) The amp was 'hi-fi' so sounded flat, so I breadboarded a bunch of distortion circuits until the owner guy liked the sound. It wound up having a softish symmetric diode clipper with a tracking clip level and a bit of differentation, just a couple of diodes and a few R-Cs in the end, with a user adjustment for how much of this mess was mixed into the main signal. Some people liked it; it added a bell-like quality to the sound, long sustain, not a hard fuzz-tone thing.

But audio tends to be boring, low-margin business.

John

It dosnt if you play a CD through it.

Actually, no commercial guitar amp has that sort of quality. Indeed, I had a Peavey Renown that tiangled at 15Khz, well, until I replaced the amp part with my own mosfet one that went to 80Khz it did.

Adding distortion can make things sound cleaner. That is sort of a mystary.

Its not all in the mind. Minds prefer distortion for some applications. For others, they don't.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

I think it is quite simple. You are used to the euphonic distortion of a tube amp that gives the illusion of a more "spacy" or deep soundstage, especially when combined with a phono turntable. I bet your opinion is also, that a black record played on a decent phono rig is superiour or sounds more lifelike than a CD. And I also bet your opinion why digital is bad is because of the "staircase" quantized and discrete values of digital compared to the continuity of analog. Unfortunatly or rather fortunately all these arguments loose ground when you compare a record player with a recorded CD from the same player. Then the phono freak cannot distinguish between the the 2 in a Double Blind Test. Which IMHO proofs the transparency of the CD as medium. Maybe we should also tap the tube amp output in the same way and then let the tube fan decide which is the real thing. I havn't heard of this test but you could do it at home NT and then have a friend help you to switch amps without your knowledge. You have to use the same listening level when recording and have a simple attenuator in front of the soundcard consisting of a simple 10k and 470 ohms -27dB attenuator.

When done come back and tell us. Otherwise better write those stories in some audiophool forum like that one of Hoffmann or other "high-end" newsgroups, not in an Engineering NG.

--
ciao Ban
Bordighera, Italy

Part of the explanation is that when a tube amp is pushed to about 1% or

2% distortion, a sinewave is distorted to produce mainly third harmonic (if perfectly symmetric) and second harmonic (if not perfectly symmetric). Solid state amps when distorting a sinewave to 1% or 2% distortion generate plenty of the higher harmonics and in most cases this is more audible.

Another thing: Apply a given signal to a tube amp and a solid state amp, with each amp having gain adjusted to give distortion of .1%, or .5%, or "barely audible", or whatever. Increase the signal strength a couple dB, and what usually happens: The distortion of the solid state amp typically worsens more than that of the tube amp does. This means that peaks that are by a small margin pushing an amplifier into distortion are distorted more audibly by a solid state amp than by a tube one.

- Don Klipstein ( snipped-for-privacy@misty.com)