Please help my audio ignorance

[W8ACR]

As I stated in my previous post, I managed to cook something in my W2IHY audio amp. I bought new IC's for it, but to no avail, still not working. Apparently, my speech amp deck doesn't have enough gain to drive the modulator directly from my Heil microphone, so I'm gonna have to come up with a new mic preamp arrangement of some sort, and I'm looking for suggestions. I don't think getting a new W2IHY unit is really worth the money, although I was quite happy with how well my previous unit worked and sounded. I have been looking at mic preamps on the internet, and there are about a million choices ranging in price from $40.00 to $10,000.00. Way too confusing. I thought I'd describe my current audio chain, and ask for suggestions on how to solve my problem.

I am using a Heil Classic mic which is a dynamic mic with two selectable mic elements, one is a wide response broadcast unit with audio response from 50Hz to 18KHz, and one is an HC-5 element with audio response from 300-3000Hz. "Sensitivity" is -72dB at 1KHz - I assume this is an indication of the audio output voltage from the mic element. The audio output from the mic is unbalanced.

My transmitter speech amp is based on a design from the Radio Handbook, fifteenth edition 1959, page 652. It has a 6SJ7 voltage amplifier which is resistance coupled to a 6N7 twin triode phase inverter. The 6N7 drives two cathode biased 6A3 triodes which drive the 811A modulators through a UTC S-9 transformer. If I hook up the Heil mike directly to the 6SJ7, I cannot get enough audio to suficiently modulate the RF final. Previously, the W2IHY unit was installed between the mic and the 6SJ7, and I had plenty of audio.

My assessment is that my mic output is just too low to adequately drive the modulators through the existing speech amp, and that I am going to need a "microphone preamp" because of the relatively low output from the Heil mic. I can either buy a commercially built unit, or roll my own. If'n I buy a commercial unit, do I need to get one with an unbalanced input, or can I run the unbalanced mic output into a balanced input? I don't mind spending a couple hundred dollars on a good unit if it will make a difference. Suggestions?

If'n I build my own, I guess I could put another high voltage gain pentode in front of the existing 6SJ7, or I could put a 12AX7 twin triode with a pot between the tube sections to get variable output from the preamp.

As I look at the commercial units available, I see that they typically have available gain of 40-80 dB or so. How much would I need? Could I drive the 6N7 phase inverter directly from the preamp and eliminate the 6SJ7 altogether?

Thanks, Ron W8ACR
skipperfam@midrivers.com

[W7TFO]

Sounds like an impedance mismatch. 

A 6SJ7 has boatloads of gain.  Don't usually need two stages of preamp unless you run a ribbon.

If you need more, a 6SF5 does a good job in the front end.

One man's opinion.

73DG

[KL7OF]

the mic is the problem....low output or impedance mismatch to the 6sj7

[W8ACR]

Are you guys saying that this mic, if working properly into the correct impedance, should not need an additional preamp?

Ron

[WA3VJB]

Among the amateur outlets that sell the Heil, their spec sheet for that model shows a choice of high or low impedance for that mic.
Have you tried switching between the two settings?  The Julius box might have been compatible with a low impedance setting, and now that Low-Z is loading down the input of your speech amp.  Switching to Hi-Z might be all you need to do.  One of the listings I saw shows that the impedance choice is at the time of purchase -- not selectable. In that case, you may have a Low-Z, creating the problem.

[Opcom]

with a nice tube lineup like that, if you can avoid using ICs, the better!

[W8ACR]

This particular model of Heil mic has a switch to go from high (30K ohm)to low (600 ohm) impedance. When I used it with the Julius box, the mic output and the audio amp input and output were all set to high impedance. I'm pretty sure I tried both mic positions diectly into the 6SJ7. I'll double check. The Radio handbook specifically said that this speech amp was suitable for crystal mics. I'm not sure, but I was thinking that a crystal mic has higher output than a dynamic mic. This is why I thought that maybe the speech amp just didn't have enough gain. I suppose I could also have a weak tube.

Ron

[flintstone mop]

OK a crystal mic is going to have VOLTS of audio out put and very high impedance compared to the better Heil Mic.
You will need a box designed to deal with microphones. Low impedance and probably -30 levels.

You're almost there......the -72dBm from the Heil is tiny tiny audio and you need more amplification. A box designed for microphone levels.Someone here should give a link to very affordable mic preamps/mixer for less than $200.....prolly less than that.

Some samples...........http://www.ebay.com/itm/ART-ProMix-Mini-Mixer-XLR-3-into-1-Mono-Mic-Microphone-Mix-PROAUDIOSTAR-/110751613314?pt=LH_DefaultDomain_0&hash=item19c94f8182

http://www.ebay.com/itm/Shure-FP42-4-Channel-Stereo-Music-Microphone-Mic-Audio-Sound-Mixer-/270774391993?pt=LH_DefaultDomain_0&hash=item3f0b6954b9

http://www.ebay.com/itm/Behringer-802-8-Input-2-Bus-Mixer-with-XENYX-Mic-Preamps-and-British-EQs-/120783788420?pt=LH_DefaultDomain_0&hash=item1c1f465984
The last one is very nice and affordable



Fred

[K5UJ]

Ron,

I am not familiar with that particular Heil mic, but are you absolutely sure it is unbalanced?  Every Heil mic I have ever seen and owned has had a balanced 3 or 4 pin XLR pinout.   Your vintage audio stage you want to work into is surely high Z (is there a grid resistor on bypass on the grid of the first tube in the transmitter speech amp, and if so what is its value?).  the heil elements in that mic vary in response but as far as I know not impedance.  it is a low Z mic I am fairly sure.   I went to the Heil website because there used to be information about all his mics and useful information about how to make the transition from balanced to unbalanced but none of that is there now    It is all about the current line of mics and working with plastic radios. 

Sounds like you have to find a mic preamp to get it to line level to drive the transmitter.  Modern preamps are usually balanced audio boxes and 600 ohms.  The rig is probably taking unbalanced audio so you will need a small Jensen audio transformer with the primary wired for balanced and the secondary wired for unbalanced into the rig.  The transformer also isolates the outboard audio gear from the rig breaking up hum and RF.  Here's a document from Jensen on ways to do the wiring:

http://www.jensen-transformers.com/an/an003.pdf

You also need to transform the nominal 600 ohm Z to whatever the value of the grid resistor is in the tx speech amp.  If you use a transformer with a transforming winding ratio it can do both that and allow for the balanced to unbalanced transition. 

I am not sure how the Heil mic element is done with respect to frequency response, so after the mic preamp and before the transformer you may have to get some sort of equalizer to get the mic sounding okay.  I'd try it first though as it might sound good with just a preamp. 

Here is a good document to look at that explains a lot about audio transformers and can help with the Z transforming problem.
http://www.jensen-transformers.com/an/Audio%20Transformers%20Chapter.pdf

Rob

[K5UJ]

This particular model of Heil mic has a switch to go from high (30K ohm)to low (300 ohm) impedance. When I used it with the Julius box, the mic output and the audio amp input and output were all set to high impedance. I'm pretty sure I tried both mic positions diectly into the 6SJ7. I'll double check. The Radio handbook specifically said that this speech amp was suitable for crystal mics. I'm not sure, but I was thinking that a crystal mic has higher output than a dynamic mic. This is why I thought that maybe the speech amp just didn't have enough gain. I suppose I could also have a weak tube.

Ron

I missed this before -- sorry about that.   Could be a bad tube but there's high z and there's high Z.   The rig might be looking for 5 megs from a xtal element.  A D104 could probably drive the rig directly.   or there might be a lower impedance point in the rig's preamp stage beyond the first tube that can accept a lower Z input.  The external mic preamp may be able to drive it.  You'll have to look at the schematic and experiment. 

[KD0HUX]

 THE HEIL MIC NEEDS A HOT PREAMP. WHEN I BOUGHT THE HEIL CLASSIC 2 RUN WITH MY RANGER BOB HEIL RECOMENDED   THE BEHRINGER 802 MIXER.

[WA3VJB]

If it's just an output problem with the Heil, I just saw in the Sunday paper that the retail chain Best Buy has a sale underway Nov.6-19, at $75, for either of two models of Shure dynamic microphones that have higher output (going by Fred's spec saying -72dBm from the Heil) and good frequency response. They don't have the same "look," but they are very popular in broadcast and stage. 

It won't fix your possible impedance mismatch, being Low-Z into a likely Hi-Z input on your speech amp.

SM58 50-15K response
http://www.shure.com/idc/groups/public/documents/webcontent/us_pro_sm58_specsheet.pdf

SM57 40-15K response
http://www.shure.com/idc/groups/public/documents/webcontent/us_pro_sm58_specsheet.pdf

This is the cheapest I've seen these mics in years. Really worth considering as a 2nd mic or a primary mic if you're not that committed to the one you've got.

[K5UJ]

Thanks for that news Paul--I did not even know Best Buy handled any halfway decent mics.  That is a really good deal on what is an industry standard stage mic.  I might get one just to see how it compares to my PR20.

[W8ACR]

OK guys, thanks for the comments. Right now, I believe there is a 2.2meg ohm resistor from the 6SJ7 grid to ground. To match the high Z (30Kohm) output of the Heil mic, is the fix as easy as simply changing that grid resistor to something like 33K, or is it more complicated than that? Yes Rob, I am sure that the mic has an unbalanced output. It does indeed have a 4-pin XLR connector - one for audio output, one for shield ground, and two for PTT. I have scanned the spec sheet for the mic and have attached it below.

I have decided that I'll probably buy a commercially built preamp, but I know that I could whip together a single tube preamp in a few hours, so I might try that first just see how it works and perhaps save a few bucks.

Thanks again,  Ron

[k0vvx]

i think i have one of deweys preamps here if you would like to give it a shot.

[flintstone mop]

I do not believe that a 6SJ7 will have enough amplification for the Heil.........
Can you build up a preamp using a 12AX7?

[K5UJ]

Ron, sorry about my doubts regarding the mic.  I had no idea Heil made such a thing although I remember when they were making that one and the lookalike shure 55.  I just never thought it was dual Z and unbalanced but it makes sense now when I think about the customers it must have been made for--guys like us.   If I were you I'd try two things--I'd call them up and get Bob Heil on the phone and ask him about working the mic into a 2+ meg impedance.  You may not know this but Bob is a vintage AM op who operates 75 m. from his QTH in Fairview Heights with his old H.W. Bandmaster.  he'd probably be able to tell you what to do, but it might be hard to track him down.  I'm not entirely comfortable volunteering someone, but I think this is okay--Don KYV has worked his modern low Z gear into his HB high Z rig--he'd probably be glad to tell you how he did it.  I run modern low Z audio gear and I know how to make the transition from bal. to unbal.  but I can't tell you exactly how to get from a modern very low Z to a super high Z.   I have a feeling chaining a bunch of transformers each with a 15:1 ratio is not the way to go hi hi.  30 K?  no problem but 2 or 5 megs?  That's why I was thinking injecting the outboard audio at some point past the rig's first audio input stage might be necessary.

[W8ACR]

OK guys, thanks again for the advice. I'm pretty sure the problem is simple, there is just not enough audio amplification with the speech amp as built. Yes Fred, I think I'm going to put together a mic preamp using a 12AX7. I'll put a pot between the tube sections to get variable outpoot, and run that into the 6SJ7. I did this once before and as I recall, it worked pretty well. I bought this Heil mic about 2001 or so and originally tried to use it with my Globe Champ 175 which by the way had a very similar speech amplifier circuit - 6SJ7 into a 6N7 phase inverter into triode connected 6F6's which drove the modulators - triode connected push pull parallel 6L6's. It initially didn't work - not enough audio to drive the modulators. I built a 12AX7 outboard speech amp and it worked well after that. After I bought the W2IHY unit, I stopped using the 12AX7 unit I had built. I had forgotten about that until this problem came up. I don't recall what became of the 12AX7 thingy. I probably sold it with the Globe Champ. Rob, I'm not too worried about the impedance matching, I should be able to figure that one out. As far as the bal/unbal issue, I just need to make sure that if I buy a commercial unit, it has an unbalanced output. Many of them do.
Ron

[flintstone mop]

Ron
We are always here to help. I'm always buggin Tom (K1JJ0 or Steve (K4HX) for crazy antenna ideas.