Fine Tuning My Tube Mic Pre - Transformers Rewired Update II

Fellow Prodigy-Pro addicts:

I am doing a plate-following single-ended 12AU7 transformer output stage for a mic pre project. However, as I was wiring it up for the final tests, I didn't notice that the 6:1 OP transformers were backwards. The preamp sounded good.

I reversed them and tried again - sounded much hotter and thinner - worse with 6:1 output than a 1:6 output.

My only theory is that the tube is loaded better with 1:6 reflecting the 10k impedance of the recorder into the OP stage of the tube and increasing the load - therefore making the OP stage more linear.

My next step is to mess with some ground reference resistors before the transformers to try to reflect a proper load into the recorder.

Any thoughts?

*deleted reply*

[quote author="beatpoet"]

I reversed them and tried again - sounded much hotter and thinner - worse with 6:1 output than a 1:6 output.

[/quote]

Are you sure you did not have it 6:1, first place? If you reversed and it is much hotter it suggests that now you have 1:6.

I checked more than once - I can't quite figure this out. My only idea is that the tube likes the higher reflected load from the recorder, and the lower output level is from the impedance mismatch.

I'll check again after dinner! I've been on this project for a few days straight and I'm gettin' burned out a bit. :?

You are saturating the core due to the fewer turns.
B max goes way up.
L , which is quadratic with resoect to turns goes way down.
More curent flows in the plate circuit, which saturates the core.

Yeah, "hotter and thinner" would be consistent with going from 6:1 to 1:6. You probably had it right the first time. Measure it!

Without taking the trannies out of the box...

@57DCR on the plate side
@8DCR on the OP side.


[edit]
My simulator shows hotter output for 6:1 than for 1:6 into 10k


http://www.gyraf.dk/gy_pd/g9/g9_sch.gif

I'm wondering what the two 10kRs parallel on the output of the G9 are for...

Your simulator? If you have the circuit right there "in the flesh", why are you running a simulation on it? :?

Grab an AC voltmeter or a scope and measure the signal level going into one side of the xfmr and coming out the other. The signal level at the output side should be about 1/6th of that on the plate side. If the signal level is higher on the output side, the transformer is hooked up backwards.

Looking over your previous messages, it seems you have the terminology of the ratio backwards, which is in turn confusing the whole discussion. 6:1 will reflect a higher impedance to the 12AU7 plate. 1:6 would reflect a much lower impedance to the plate.

[quote author="NewYorkDave"]
The signal level at the plate side should be about 1/6th of that on the output side. [/quote]

Hey Dave,

Did I read it right, or is it my English?

Here are some quick output tests. This is what I mean by 'thin' sounding. My -3dB are technically @ 130hZ - 8.3kHz.

The low end dropoff starts at about 80Hz - Much higher than I intended, especially with slightly oversized K and output caps. I did use electros for the OP - maybe a mistake there.

I put in some switchable output load resistors, that's what's being referred to in the graphs.

Next I will measure the signal on each side of the xformer - a good idea.

Good catch, Mark! I made a typo and transposed "plate" and "output" (although, what I meant to say is made clear by the sentence that follows it).

Also, turns out my OP transformers are closer to 3:1 after the signal test.

Pays to test them. :?

NYD, I see your point about mixing it up - I had been working on it waaaay toooo loooong without a break.

Well... This begins to explain:

1) High output
2) The high/low end signal loss

Too bad they are the only outputs I have left! Period!

[edit] This pre is supposed to go out to a session in Wisconsin on Monday; hence the rush. I'm thinking it's almost good enough to record the rock guitar that it's intended for...

I can't think of anything right now to fix it :? other than put the transformers back in backwards and re-test the freq. response.

[edit] [edit] [edit] I think it's time for bed pretty !@#$%#$ soon. Cheers. :guinness:

Just to get the terminology straight: in an A:B transformer, A is the primary and B is the secondary.

If the transformer is 6:1 turns ratio, then its impedance ratio is 36:1, and a 10k load will look like 360k to the 12AU7, which will be happy with that.

If the tranny's backwards and 1:6, though, a 10k load will look like 10k/36m or about 278 ohms. The 12AU7 will not be happy at all.

EDIT: Oh, and don't go by the DC resistance. That's only valid if the primary and secondary are wound with the same diameter wire.

Peace,
Paul

I inserted audio in parts of the chain....

Looks like a HF drop in my input tranny, a 15095A. Why?

The problem is only worsened by the following stages... There seems to be a low end rolloff in both the input and output transformers :?

What I don't understand is the high frequency rolloff. What causes this? Capacitance to ground? Inductance somewhere?

How do I milk more bass response out of the output Xformer if possible?


Any ideas on how to get a better bass response out of this?


Slight HF drop- added to stage 1 = rolloff -3dB @20+k


Looks decent - x'cept for that dip below 80Hz


Through input xformer for comparison...

What is the source impedance with which you are driving the input transformer?

What is its load?

Peace,
Paul

The input stage is lifted from here:

http://www.jensen-transformers.com/as/as020.pdf

150R driving impedance (mic level) driving a 1:10 wired 15095A and a 150k load w/a 1k grid stopper.

15k/150 = 100
1.5k load X 100
=150k load R

I wired up for 150R as per the can. Maybe I got it wrong -

the can says:

"150R -> Strap 1-4......3-6"

I assume that the 150R input is 1 & 6. Or are these two different 150R windings?

Did I ground one of the hots? I don't think I did. For my next test, I'm going to use another transformer before the input to get a precise 150R input Z for measurement if possible.

:?

I found an old pair of BPIs - 15k to 1k that are going to work fine I think. Now, to figure out the HF loss at the input. Any suggestions?

If it sounds better backwards, why not use it like that?

A quick update.

I rewired the input and output transformers for 300R, maybe less wind inductance/capacitance... And the frequency response flattened out.

Now the bass sounds all tubby n'shit... Any ideas? It's distorting!

I'm about to add a NFB stage to chill the low end out a little bit.

Here's a mono clip through a small diaphragm CF condenser - about 6 feet in front of the set. The set is a slingerland - all the cymbals are either broken, cracked, or got dished out on my dishing stump by me and a 10lb hammer.

http://www.geocities.com/microphoneland/Audio/TubbyDrums.wav (you might have to cut and paste)

Clip weighs in at about 750k memory.

Here's a stereo clip. This stereo track is actually both channels pointed in the same direction panned hard L and hard R.

http://www.geocities.com/microphoneland/Audio/TubbyDrums2.wav

The hard L channel is the updated :? 300R:15k input (single winding) and the 15k:500R out (also single winding)

The hard R channel is 150R in and the old 3:1 out (with the supposed bass loss)

The frequency response of L is more accurate according to sine and square wave sweeps, but:

I get this sloooowly drifting DC offset when I first apply the sweep that returns to zero eventually. I think this DC offset applies to the bass frequencies.