Are these transformers worth keeping?
- Thread starter JohnWatkins
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gemini86
Well-known member
Are they step ups? If so, put em up on ebay and get at least 10 bucks each out of them. They'll sell, for sure.
or if that's not worth the effort, you could just send them to me and I'll make sure they get put to use somewhere.
or if that's not worth the effort, you could just send them to me and I'll make sure they get put to use somewhere.
ruffrecords
Well-known member
According to the schematic they are 150:10K step ups so they could be OK for use in a tube mic pre.
Cheers
Ian
Cheers
Ian
JohnWatkins
Well-known member
Forgive the noob, but does that mean the expected impedance of a microphone at the input is 150ohms and it steps it up to 10k for the eq and preamp?
sahib
Well-known member
Yes.
It also steps up the voltage too by the square root of the impedance ratio. So that transformer has a 1.8 turns ratio.
It also steps up the voltage too by the square root of the impedance ratio. So that transformer has a 1.8 turns ratio.
I am not a big fan of transformers, but those will work Ok with modern opamps too.
JR
JR
1:8sahib said:
JohnWatkins
Well-known member
So, for example, 1.5v becomes 4.24v? If you were headed in to an opamp from there, would you resistor that back down before going in or leave it to get higher gain? Sorry to derail this into electronics 101, but I've got the learning bug!
10k/150 = 66+ sqrt 66= 8+ so 1.5V input = 12v or so.JohnWatkins said:So, for example, 1.5v becomes 4.24v? If you were headed in to an opamp from there, would you resistor that back down before going in or leave it to get higher gain? Sorry to derail this into electronics 101, but I've got the learning bug!
Optimization for noise falls under NF (noise figure) analysis. NF is comparison of ein (equivalent input noise) of a given circuit vs a perfect noiseless circuit.
OK without jargon, you want to select an op amp or gain device that is quiet wrt 10k source impedance. Back in the day when FET input opamps were noisy, this involved calculating the input noise current contribution from good bipolar opamp times 10k added to the active device noise voltage. Modern high performance FET input opamps are low noise too, with almost insignificant noise current, so most high performance low noise FET input opamps should be fine.
JR
JohnWatkins
Well-known member
Thanks for the explanation. I have much to learn.
ruffrecords
Well-known member
JohnWatkins said:
Just to put this into context, this is a mic transformer so it is not expecting to have to handle line level signals like 1.5v and probably won't. It's ty.pical input level from a mic would be a few millivolts which it would amplify by about 18dB.
Cheers
Ian
Yup a rule of thumb for audio transformers is that it takes more transformer to handle the product of signal voltage "and" LF bandwidth... So dropping to lower frequency for same level or expecting higher level for the same LF bandwidth requires more iron.
This is one of the several reasons that premium input transformers are bigger and cost more.
You can tell from the picture that those are not premium jensens... While it's possible to make an audio transformer large and lousy, they rarely enjoy enough success to do it more than once.
JR
PS; Transformers are generally described by turns ratios rather than having gain, since the output power is pretty much the same as the input power, just transformed in voltage and impedance. For voltage amplification it is too convenient to not think of it as voltage gain, just beware of crusty old pedants if you utter that your transformer has gain out loud. 8)
This is one of the several reasons that premium input transformers are bigger and cost more.
You can tell from the picture that those are not premium jensens... While it's possible to make an audio transformer large and lousy, they rarely enjoy enough success to do it more than once.
JR
PS; Transformers are generally described by turns ratios rather than having gain, since the output power is pretty much the same as the input power, just transformed in voltage and impedance. For voltage amplification it is too convenient to not think of it as voltage gain, just beware of crusty old pedants if you utter that your transformer has gain out loud. 8)
sahib
Well-known member
abbey road d enfer said:
Apologies. Speed writing.
ruffrecords
Well-known member
JohnRoberts said:
I think we are lucky in this group in that most of our old pedants are quite mellow.
Cheers
Ian
okgb
Well-known member
How about " free gain " ...........................[ yeah I'm not serious ]
How about free bird?okgb said:
JR
Agreed. In fact I think we should rename this section Mellow pedants.ruffrecords said:
okgb
Well-known member
Mellow Southern pedantics ?
JohnWatkins
Well-known member
Ok, I pulled one of them out and I'm identifying the windings (noob learning exercise). It looks like in the schematic that the primary has a center tap and the secondary doesn't, so that makes it pretty obvious since there are three terminals on one side of the can and two on the other. Also, the schematic shows that the center tap is not connected to anything and on the circuit it goes to a resistor that is absent (I guess they used the board for multiple models; there are a lot of empty component spots) so that checks out.
I'm getting 20R on the primary and 700R on the secondary. If you didn't have the schematic, how would you do the math to determine that it's a 150:10k step up just with that info? Or would you feed it power and use the voltage on the other end to do that math?
I'm getting 20R on the primary and 700R on the secondary. If you didn't have the schematic, how would you do the math to determine that it's a 150:10k step up just with that info? Or would you feed it power and use the voltage on the other end to do that math?
