Phase spliiter or EQ?

[doctorcilantro]

Hi all,

I own a Sunfire Vacuum Tube preamp and it has a simple EQ ("Contour Control") with treble and bass, which is activated by a switch. The driver stage has (3) 6922 and the manual states that one of these is for Contour. Can I run this pre without this tube for the Contour if I don't use the EQ. Not knowing a lot about this stuff, I was wondering if this third tube is just for EQ are what I have called a phase splitter.

I called Sunfire, but as they don't make the pre anymore, the tech really couldn't tell me a lot at all, and I decided not to push the issue.

The manual is here: http://www.sunfire.com/pdf/Classic%20Tube%20Preamp%20Manual.pdf

thanks for any help!

jon

[russ69]

I'm no expert but I think it would be best to run all 3 tubes, as the designer intended. Some expert will pop up shortly and tell why but most pre-amp sections have 3 tubes.

Thanx, Russ

[WMcD]

I couldn't say without seeing a schematic. Looking around the web I don't find one either.

My guess is that the amplifer requires all the tubes to work.

Here is an interesting link to tone controls on tube preamps in general.

http://www.angelfire.com/electronic/funwithtubes/Amp-Tone.html

It shows that balancing controls are used with a vacuum tube to drive the passive filter components. It does not show a bypass switch. None the less, I expect that a bypass (flat) switch bypasses the passive filter components and not the vacuum tube. The latter would make the tube un-necessary, in theory. I suspect, without knowing, that is not the case.

An additional issue is that much of the circuitry is expecting the tube to be there for bias and power supply voltages. So it ain't gonna work very well without the "missing" tube.

I see that there is an option phono (phonograph record player) amp circuit. There, the phono amp (or lack of it) is bypassed. Perhaps you were thinking in that direction.

= = = =

You mention a phase splitter.

A phase splitter circuit is used as a driver for a push-pull tube amplifier. Essentially, there are two "final" amplifier tubes connected to either end of the primary of the output transformer. One "final" has to be fed with signal of reversed polarity. The phase splitter is a scheme using a single tube and circuitry with two "outputs" One is inverted polarity and the other is not. Those circuit outputs (a pre-amp within the push-pull power amp) feed the respective output tubes.

This is not intended to affect frequency response.

Best,

Wm McD.

.

[doctorcilantro]

Thanks. So would it further important to match the Contour tube to the other two?

On another note, I just adjusted the trim pots on my DAC1 output. Previously, going to my mono-blocks with an input sensitivity of 1v, I had .880v at about 2 o'clock on the Sunfire preamp master gain. I didn't have enough headroom and never could use the volume above 10 o'clock.Carver discusses a few disadvantages of this situation in the manual, so I re-adjusted the system to have:

DAC1 outputs 265.5mv per channel into Sunfire

Sunfire outputs to Quicksilver mini-mites:

9 20.5mv

12 122mv

3 323

MAX .995v

Things sound sweeter already and I really enjoy the greater control of the gain (more steps?)

So......a friend of mine asked:

"A max input signal of 1.2 Vrms shouldn't clip the inputs on your
Sunfire....I didn't look at the manual, but it should handle that
signal level easily.



Where do you run your volume control, though? My measurements of
several preamps and the pre-out of an NAD integrated showed that the
"zero gain" point is usually around 1 to 2 o'clock.....so if you run
the knob lower than that, the preamp is actually attenuating the input
signal, sending a weaker one to the power amp!



(The specs on the Sunfire show the line input gain to be 12 dB, that's
a voltage gain factor of 4, so it would drive the outputs to 8 Vrms
with the knob cranked to the max with a typical CD player input....and
that's what my Musical Fidelity CDPre24 was spec'ed at, within a couple
of tenths Vrms.)



For the life of me, I fail to understand the decisions about the
relative gain of devices in the chain these days. It does make sense to
increase the voltage out of a CD player or DAC.....thus placing the
signal higher above the noise floor.....versus what typical old phono
preamps put out, which was usually less than 1 Vrms on a test tone, or
tape decks that were designed to put out 0.775 Vrms when the VU meters
were at 0 dB VU. But those were not hard upper limits as is 0 dBFS in
the digital realm...there was headroom above the 0 dB references in
those days, because 0 dB wasn't referenced to a constrained maximum
level.



I really wish I knew why makers construct power amps that clip at 1 or
1.2 Vrms when preamps can put out 8 to 10 Vrms. The current generation
Quad SS amps clip at the old 0.775 Vrms standard that was typical in
European gear for many years....that's ~ 8 dB less than a CD player
puts out! Many users end up having to add fixed attenuators between
non-Quad preamps and the 606 or 909 to avoid hair-trigger volume
control.



Power amps could be designed for 8 to 10 dB less gain in most cases and
probably be even more usable as a result--finer volume control over the
range of CD's that are mastered all over the place these days.



Maybe it's to give the user the perception that if the system is loud
with the volume control at 9:30, it's great because that means there is
a lot of headroom available.......when in reality, it just means you
better not go above 1 o'clock or you will damage your tweeters in a
hurry!"

dc