Re: Toob sound?

On 7 May 2005 02:38:52 -0700, bigcat@xxxxxxxxxxx wrote:

>John Larkin wrote:
>> On Fri, 06 May 2005 23:20:20 -0400, John Popelish
><jpopelish@xxxxxxxx>
>> wrote:
>>
>> >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.
>>
>>
>> Or generalize to a polynomial.
>>
>> Tube sound seems to require the mere presence of a tube in the gain
>> path. Single-ended, push-pull, class A or AB, small/large signal, all
>> of the permutations create "tube sound." So good luck trying to
>> quantify it.
>>
>> John
>
>its the output stage thats the issue, not the rest.

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


> 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.

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

John


.

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