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Old 23rd March 2013, 04:36 PM   #11
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by Vinylsavor View Post
Hi Cassiel,

no pro equipment needed really. I would highly recommend to get a scope and a signal gen. Used scopes can be found for small money. Signal generators are cheap too. For something like 200-300 Euros you can get both a scope and generator. This will give you a lot of insight and help to understand things.

Best regards

Thomas
I agree.

Even downloading the free HolmImpulse software for your computer and a cheap soundcard with a mic input will do the trick.

HolmImpulse will do a complete sweep across the audio spectrum in about a second and display the results as a graph on your PC.

You need to establish a baseline before making changes so you know where you are at and what changes do what. Subjective listening tests are not the way to go.
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Old 24th March 2013, 05:11 PM   #12
Cassiel is offline Cassiel  Libya
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Quote:
HolmImpulse
I have installed the program and I reckon it will take me a year to understand how it works.

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Originally Posted by Loren42 View Post
You need to establish a baseline before making changes so you know where you are at and what changes do what. Subjective listening tests are not the way to go.
My baseline is a great sounding preamp with zero noise but (there's always a but) bass response could be better. And about what changes do what... I can say that doubling the value of my output caps did nothing. Anyway, I'm fed up - I hate this A/B thing. Sometimes the changes are obvious and easy to spot but when they're not... it's frustrating. It's like looking for a missing person. I think I'm gonna hire a detective (oscilloscope) to look for my missing bass. Either that or accept you can't have it all.
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Old 24th March 2013, 05:42 PM   #13
GoatGuy is offline GoatGuy  United States
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say that doubling the value of my output caps did nothing
I think you're "hearing phantoms" and preferences. The cathode cap (47uF) gives a -3dB rolloff of 6.3 Hz. Super. Not the problem. The output cap (2 uF) driving a 100K load, 0.7Hz. Clearly low enough!

Before you go all oscilloscope crazy, let me ask, what again is the point in even having a preamplifier that has a thru-gain of 1.28x ??? The best answer would be, "to convert from high-Z source impedance to low-Z input impedance of my amplifier" Because otherwise, there isn't much that the preamp with a gain of 1.29 is actually doing.

WAIT! I think I may just have gotten "it"!

This preamp circuit is a phase INVERTER. When your speakers are supposed to be punching OUT, they're now going to be sucked IN. And vice-versa. This very well could be the problem, in a nutshell.

Simplest test in the universe: reverse your speaker cable polarity. Well, not simple if you have some kind of standard connector other than banana-plug. But this would do the trick to test the polarity issue. If your amplifier is worth anything at all, it will symmetrically amplify the signal, so it itself doesn't "care" what the signal looks like coming in.

IF this fixes things, then there are two fairly obvious "fixes" that come to mind.

1. Feed anode of U1 to grid of U2, which has a pair of 22K resistors, one on anode, one on cathode in the classic "phase splitter" configuration. Tapping the plate with the output DC blocking capacitor as in your original drawing will deliver a PHASE NORMAL signal to your amplifier. Feeding the cathode (through a DC blocking cap) through the R2 NFB resistor preserves the basic circuit topology and NFB idealization.

2. ... or get a small interstage coupling transformer, and replace both the 22K plate load resistor and the output capacitor entirely. The beauty is ... if you get a center-tapped type, you can have both the positive output and the negative feedback at the same time. No additional tubes. Capacitor phobic types will see this as a great gain. You do have to have a nice transformer though, to get down to the 20Hz listening ideal.

There you go. I don't think putting bass "shaping" circuitry (DF96's tone control comment) is anywhere near as important as just re-reversing the phase to be coherent with the original sound sources.

GoatGuy
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Old 24th March 2013, 06:13 PM   #14
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Originally Posted by GoatGuy View Post
WAIT! I think I may just have gotten "it"!

This preamp circuit is a phase INVERTER. When your speakers are supposed to be punching OUT, they're now going to be sucked IN. And vice-versa. This very well could be the problem, in a nutshell.GoatGuy[/b]
Haha! You crack me up

jan
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Old 24th March 2013, 06:27 PM   #15
Cassiel is offline Cassiel  Libya
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Well, thanks for the suggestions GoatGuy. It could be that. I will have to check that out. As for the reasons of having a preamp....these are simple enough to understand: selecting different inputs plus a remote volume control.
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Old 24th March 2013, 09:03 PM   #16
zelgall is offline zelgall  Canada
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In tube cad, try changing units to 1 and leave triodes at 2. You're using a single paralleled 6sn7. It won't solve you're problem but it will change the sim.
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Old 24th March 2013, 09:40 PM   #17
Cassiel is offline Cassiel  Libya
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No. It can be confusing but that's how it works. The data is OK.
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Old 26th March 2013, 08:59 PM   #18
jgf is offline jgf  United States
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Cassiel, I think you may have been on the right track with your initial post, that is, look at the power supply. What are the values used in the CRC filter?
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Old 26th March 2013, 11:41 PM   #19
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As GoatGuy said in post #13, it is clear that this circuit is level down to at least 20 Hz. The problem must lie somewhere else. (By the way, I cannot seem to find the value of R2. Dirty spectacles?)

Is the amplifier it is compared to not bass-heavy?
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Old 27th March 2013, 11:56 AM   #20
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remove r2

replace r1 with a wire. input resistor 330K instead of 1M, remove ck.

rk 680 ohms, rl 470k coupling cap 2.2uf poly (no wima or wima copies)

anode resistor 22k ok.
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