Impedance Matching Transformer

[W7SOE]

I have put together a little audio amp from a Canakit product, a little SS 25W amp.  It has peak output at 4 ohms.  I need to sent this audio to the modulator deck which wants 500 ohms.

So, I thought I would use a speaker matching transformer in reverse.  When I try this the transformer acts like a dead short, what am I missing?

Rich

[WA1GFZ]

series cap

[W7SOE]

Insert Headslap.  I see now, as I have actually hooked up the scope, the DC offset.

Thanks

Rich

[WBear2GCR]

Actually, if you only need to supply a 500 ohm input on the mod deck, and there is sufficient swing from the SS amp, no matching device is required. Otoh, going from "4 ohms" up to 500 ohms via a step up transfomer will give you extremely high swing.

I don't see a need for a blocking cap at the input to the low Z side of a matching transformer. Depending on what your driving on the mod deck side, you may or may not want to block DC from the iron.

Try running the matching iron with a 500 ohm load and see if it works - you might have a miswire, or a bad transformer.

              _-_-bear

[KL7OF]

ground return?   center tap?

[WA1GFZ]

SE to PP conversion. He would need 2 amps out of phase to eliminate the transformer. Positive voltage offset would love to motivate the mod tube resting current.

[W7SOE]

getting low output from the SS amp.  I have the condensor mic hooked directly to its input (with series cap and pull-up).  It looks like I may need a mic pre-amp.  Perhaps a simple op-amp circuit......

Ideas?

Thanks


Rich

[WBear2GCR]

Since the SS amp probably has no DC offset at the output, no blocking cap needed?

Put up the schematic, and/or the specs? The specs ought to tell the gain, and the expected input signal level...

If you need to get up to line level, then you'll need a mic amp circuit. The Handbook doubtless has one, and there are whole bunches online, not to mention "all in one" chips that do the job very well. Probably National, Analog Devices, Texas Instruments all have those all in one jobbers.

In a pinch any tape recorder with a mic input will do the trick.

                        _-_-bear

[KL7OF]

I need a refresher on your rig..  What are your mod tubes?  How are they biased? Is the 500 ohm input to your mod deck a transformer?  does your condensor mic require phantom voltage...?   

[W7SOE]

I need a refresher on your rig..  What are your mod tubes?  How are they biased? Is the 500 ohm input to your mod deck a transformer?  does your condenser mic require phantom voltage...?   

I have attached a rough schematic.  Audio is fed into a Stancor "Poly-Pedance Line Driver" transformer which wants 500 ohms.

I have not been able to get much output from this little SS amp so I built a simple op-amp mic pre-amp.  Seems to work, needs tweaking....

Yes, the condesor mike has a five volt pull-up on the output.

thanks

Rich

[flintstone mop]

You're probably getting a little confused with all of the responses here, BUT the condensor mic might need "phantom power" 45 volts from the mic pre to make it work. The power out of your amp should be enough. You just need to match and convert to push-pull audio for the mod tubes. Even a transformer used in reverse will work. Coupling cap for your DC offset and a the other side of this "matching transformer" center tapped for phase inversion and adding any biass to the mod tubes.
You're almost there.
Fred

[W7SOE]

I am not sure what phantom power is but the mic spec calls for a 10V max pullup on the output with 2V nominal.......


Thanks

Rich

[KL7OF]

Do you have an instruction sheet for the poly-pedance xfmr?   those xfmrs should have green leads to tube grids and yellow leads to bias or ground and to complete the grid circuit connect from lugs "L" (the two center lugs on the lower edge of the panel) to the desired ratio lugs... I am trying to scan and post an instruction sheet for the poly pedance 4765 but no joy so far...Also .....some condensor mics require a 47 volt source to operate .... that is called phantom voltage..

[nu2b]

Sounds like electret mic, not condensor!

Bias the mic with a 10k resistor to +9v.
Take the output at the mic (+2V)  through a 1uf or so cap to
the input of the pre-amp.
Mic output should be approx 3 to 10 mv rms.

[W7SOE]

yes, electret, not condenser.

rich

[KD6VXI]

I am not sure what phantom power is but the mic spec calls for a 10V max pullup on the output with 2V nominal.......


Thanks

Rich

Rich,

Phantom power is using the same line to carry the audio and the DC to power the circuit / mic / what-have-you.  It's a carryover from the networking industry...  A LOT of remote wireless products use 2 of the 4 unused wires in the 8 wire CAT-5 as phantom power, as well.

Basically, saves running two wires up the chain, you carry the DC and the signal on the same wire.

--Shane

[Steve - WB3HUZ]

An electret mic is a condenser mic. It's just self polarized with an electret coating (doesn't need phantom voltage for polarizing). They will still need either local or phantom voltage to power the preamp.

[nu2b]

Yep,
But two parts:
The condensor mic is essentially merged into the gate cap of a fet.
The fet then acts as a low-Z buffer.
You still need to bias the fet via the 9v and the series resistor.

[WBear2GCR]

Looking at your schematic, you'll only need to do the following:

- supply ~2-3 watts to the grids of the 811A
- supply the requisite swing to the grids of the 811As to fully drive them

Now, the input to the grids will be supplied by the transformer. It is important to know the turns ratio - not the impedance ratio - of the input transformer (the 4765). The RCA book calls for a bit less than 200volts of grid voltage in class B on the grids (two tubes).

IF you had a 1:1 ratio transformer then you'd need to supply 200volts to the input.

If you had a 1:10 ratio, then you'd need to supply 20 volts to the input.

The amplifier has to merely swing that much voltage at the output - if the input is actually 500 ohms the output impedance of a normal audio amp is not a factor, since it thinks it is being run without much of a load being designed for 8ohms or 4 ohms.

The grids, except when being driven positive look like a very very high impedance - so the driving transformer looks virtually unloaded until that point - and then it's not a big load in Class B2 (driven positive) since that takes only 2-3 watts to accomplish!

The whole thing depends on the step up voltage ratio of that driver transformer and how much voltage swing is available from the little audio amplifier. That voltage swing is simply equal or just slightly less than equal to the power supply voltage of the amp.

In practice you might need to provide some resistance across the input terminals of the 4765 to give the amp a non-reactive load, or similarly across the grids or from the grids to ground of the 811s for the same reason.

           _-_-bear

[KL7OF]

The 4765 has 7 different ratios....(primary to 1/2 secondary)
Terminal(s) 1...(the two outside lowest)  = 2.6
Terminal(s) 7...(2nd row center two)      = 5.0   
1 and 7 are the highest and lowest ratios available.
If I can figure out the software for the new printer/scanner with Vista(ugh) I'll post the 4765 data sheet here...Good luck   Steve

[W7SOE]

The 4765 has 7 different ratios....(primary to 1/2 secondary)
Terminal(s) 1...(the two outside lowest)  = 2.6
Terminal(s) 7...(2nd row center two)      = 5.0   
1 and 7 are the highest and lowest ratios available.
If I can figure out the software for the new printer/scanner with Vista(ugh) I'll post the 4765 data sheet here...Good luck   Steve

Steve,
   That is VERY helpful.  I have not been able to find any information on that transformer.  When I can I will take a look at the jumper settings on it.

Rich

[W7SOE]

Looking at your schematic, you'll only need to do the following:

- supply ~2-3 watts to the grids of the 811A
- supply the requisite swing to the grids of the 811As to fully drive them

IF you had a 1:1 ratio transformer then you'd need to supply 200volts to the input.

If you had a 1:10 ratio, then you'd need to supply 20 volts to the input.


           _-_-bear

Bear,
   Very helpful.  I will try to find out what the ration of that trans is and also what the swing of the SS amp output.

Rich

[WBear2GCR]

In case you can't quite make the swing - and determine that empirically with a dummy load resistor across the secondary of the 811s or at the grids of the 811s (or without 811s but with a suitable resistances to ground to simulate the 2-3watt grid draw) - you can always add an additional transformer to the 500 ohm end of the present input transformer. Keep in mind it needs to be rated with enough "watts" to not saturate when providing the requisite power.

A cheap way to go is to find a "70volt" transformer that is used to run multiple speakers in places like stores and offices. They have speaker impedance (16-8-4 ohm) taps on one side and "70 volt" taps on the other (some are multi-tap affairs). You'll probably want to put a small power resistor across the primary of the present transformer if you strap the two together - maybe a ~2kohm approximately at 10 watts would do more than fine. Just to provide some damping and resistive loading.

The other alternative is to add a tube stage inbetween the source and the driver transformer - which might be your best solution anyhow. That way you can make up the voltage swing via the gain of the tube you'd add.

Something like a 6SN7 might have enough watts in each section to do the job.
A pair of 6W6 or 6V6 is more than sufficient. Many different tubes can be considered, it's not terribly critical. I'd feed it with a line level matching transformer, that will do the phase inversion and give you PP drive so there is no DC offset to worry about on the primary of the present input iron.

Or you can use a single tube and do a "Parafeed" set up, that's like a modified Heising. Simply put a choke on the plate of the tube and a cap going to the output from the plate, in this case going to the primary of the present input transformer. Again, feed the new tube with a transformer if you like.

Another neat alternative to the present input transformer would use two tube stages, the first one going to the 811s would be cathode followers, and the one before that would provide the gain and voltage swing. The benefit to the cathode follower is that it can supply power to the grids of the 811s.

          _-_-bear

[AB2EZ]

Question:

Looking at your schematic... it appears that you are using the two resistors across the filament transformer to provide a path to ground. These resistors will produce negative feedback on the cathode of the tube, and will also produce a positive bias on the cathode. Maybe that is what you intended.

Looking at the data sheet for the 811A, it would appear that you don't want any bias voltage between the cathode and the grid for normal Class B operation. I'm not sure you want the negative feedback either.

You may need to use a center tapped filament transformer for this application.

Likewise, you need to reference the grid of each 811A to ground. This can be done by placing a 10k ohm resistor from each grid to ground. In your schematic, the grids appear to be just floating.

Best regards
Stu

[W7SOE]

Likewise, you need to reference the grid of each 811A to ground. This can be done by placing a 10k ohm resistor from each grid to ground. In your schematic, the grids appear to be just floating.

I suspect this is just a mistake in my schematic, I need to pull the mod section out and have another look.

I am not sure what path to take here.  I have this little SS amp but it seems to need a pre-amp, the electret mike doesn't seem to have a big enough signal to drive it.  Then there is the question of output voltage, it is maybe 10 Vpp, probably not enough to drive the mod section, or is it?  This rig was built (probably) in the late fifties early 60's so I think it was designed to be driven by a tube amp which, I assume, would have large voltage swings.  I need to take it out and try to figure out what the turns ratio is on the "poly-pedance" transformer. 

If I can figure out the software for the new printer/scanner with Vista(ugh) I'll post the 4765 data sheet here...Good luck   Steve

Steve that would be very helpful.

The other alternative is to use the JVII for an exciter AND audio amp.  I am in the middle of re-working its audio section now.

How do others drive the grids of the mod tubes using SS amps?

73

Rich