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Author Topic: want better fidelity  (Read 5117 times)
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w6xr
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« on: November 03, 2009, 11:13:52 PM »

I've been on AM for 42 years and now I want to have better fidelity with my stock Collins 32V3.  There are volumes of ideas I've found, however, where does one start.  What additional equipment should I start with besides a replacement for the D104.  Really, what I need is good advice as to what to buy, build or acquire.  What's first and where to I go after that?

Hopefully, I'l end up with a

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Todd, KA1KAQ
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« Reply #1 on: November 04, 2009, 12:10:13 AM »

What additional equipment should I start with besides a replacement for the D104. 

Provided it's a crystal D-104 and the head is good, why would you want a replacement? Even without modifications, the D-104 is a pretty decent match up for the 32V series. By simply changing the grid input resistor for the first audio amp/mic input from its stock value (1 Meg as I recall) to something in the 5-10 meg area, you'll fill out the modulation with more lows and keep it smooth. You could hang a bunch of boxes off it for outboard processing/distortion, but if you're going to get that deeply into it, why not build up a bigger rig? Then you can build the audio section to your requirements.

Not that you can't do more to the 32V, but it's a bear to work on inside, a lot of stuff crammed into a small (for the time and size of parts) package. Change that resistor, check and replace caps and out of spec resistors, and you'll have a nice sounding transmitter. I've got a stock V-2 that regularly gets unsolicited 'nice audio' reports, and I've yet to even change the grid input resistor.

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« Reply #2 on: November 04, 2009, 12:44:18 AM »

looking at the schematic would be good for those here to suggest things. I don't see it on the CCA website, 32V-3. Usually a good place to start is to take a hard look at the modulator output stage and use concepts from hi-fi amps where applicable. But it has to be studied to understand what any shortcomings are.

If it is like the 23V-2, meaning it has a similar modulator, my first question would be this:

Have you connected it to a dummy load and loaded the TX to rated power or slightly less and done an audio sweep from 20Hz to 20KHz on it in order to get an idea of the shape of the undistorted power band of the modulator? Perhaps it sounds ridiculous to test with such a wide sweep but that will give you an idea of what you already have to start with, to make it flat as possible over the (voice) range you want. The wider the undistorted power band of the modulator, the better the fidelity will be in the middle of it where the voice lies.

I notice the 32V-2 has a 500 Ohm tap on the mod xfmr, and also there is no negative feedback taking advantage of the tap there. That would be something to think about adding. If possible it could be taken all the way back to the cathode of the second stage of V201 through a 0.1uF 3KV ceramic cap, then a 1 to 5 meg resistor. To use neg FB you have to increase the gain elsewhere. The resistor value can be adjusted to give the same gain as before, and it can be bypassed with a 1 to 5pF cap with value selected to level out the "tilt" of the freq response. I have attached a schematic from a 50W hi-fi style public address amp to show what I mean about the neg FB loop. It inly needs a 18K resistor because it's getting FB from a 16 Ohm winding, not a 500 Ohm. The lower Z there also calls for a bigger cap across the res. so the 100pF (47pF in some models). The other schematic is a PP-PAR 807 amp. The point there is showing neg FB coming from a hi-Z place, the plate. So the values are different - 2M Ohm and 23pF. (but on that amp, disregard the bizzarre frequency shaping network at the input stage) It might be good for stability also to put a 500pF 3KV cap across the mod xfmr primary. Neg FB it will compensate the gain for the loss caused by the 500p cap. All that has to be experimented with. With neg FB, the audio response can be flattened alot and distortion reduced.

What do the plate voltages on the preamp and driver do when the modulation is present? Sometimes a larger B+ decoupling cap (C204 maybe) can help keep them from following the B+ voltage fluctuations during the overall audio envelope as closely.

What does the bias supply look like at the CT of the driver xfmr with 100% modulation? There is only 2uF there. The signal should be DC with no waveform on it when the mod goes to AB2.

About the driver transformer, is it large enough so the DC from the driver plate is not magnetizing the core too much? Would it be worth it to substitute a 6K6 (more voltage gain to make up for adding negative feedback) or something in there with a 10K resistor for its plate load and then capacitively couple the drive from its plate to the driver transformer with a 4-10uF cap and eliminate that DC?

C206 the cathode bias bypass on the driver is quite small. That stage is really a power stage too, just a small one. A hi-fi amp would use something more like Rk(Ohm)*Ck(uf)=20,000, so a 33uF would maybe help reduce low frequency degeneration. The values would be different for substituting a pentode driver.

For stability sake in the mod power stage, also consider adding a 50 Ohm carbon resistor in each plate lead and a 1K resistor in each screen lead. Things that were stable when there were no extra complexities like feedback, etc, can get a little wierd when it is added. A whole lot depends on the iron. But the collins usually have good iron.

Just throwing some things out there. You must have thought about this stuff if you been on AM for so long.


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« Reply #3 on: November 04, 2009, 11:35:55 AM »

What better fidelity characteristics are you trying to achieve? Have you gotten reports of restricted audio, or not enough audio? Do you monitor yourself off the air that you're not hearing better fidelity?
As others said, if the D-104 mic element is good, then you might have to start sweeping the freq response of the audio circuits of the V-3. When was the last time you performed any maintenance on your TX?
I got in a rut with my R390A and accepted it as a good receiver. One day I went through alignment and performance check and replaced all caps in the P.S. on the audio module and it was like a new receiver. It's on 24/7 during the Winter months.
Let us know what's going on at your station.

Fred
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Fred KC4MOP
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« Reply #4 on: November 04, 2009, 02:03:44 PM »

I've been on AM for 42 years and now I want to have better fidelity with my stock Collins 32V3.  There are volumes of ideas I've found, however, where does one start.  What additional equipment should I start with besides a replacement for the D104.  Really, what I need is good advice as to what to buy, build or acquire.  What's first and where to I go after that?

Hopefully, I'l end up with a



Hi Natan, hopefully your truncated posting doesn't mean you got across the B+ or something, writing while you maybe were poking around in there.

First of all, the transmitter, stock, sounds pretty good. But a few changes can significantly improve both the treble and bass response.

The audio stages are easy to get to and not difficult to work on, that will encourage you too. 

I don't have the values in front of me, but generally,  I decreased the bypass capacitance, and increased the coupling capacitance in the 6SN7/6SL7 stages.  Just get some values and do some substituting, checking yourself and perhaps making some recordings off www.globaltuners.com.  Just pick a receive site that can hear your signal on whatever band and time of day you're operating.

These recordings will help you track progress as to whether you've improved or degraded toward the goal of whatever "sound" you've got in mind.

I made the modifications to my 32V2 about 30 years ago, and have left the transmitter alone ever since. I did the same mods to a 32V3 to the same level of success. My brother's got that transmitter now and he gets good audio reports using an Electro-Voice RE-20 or a 664.

Here's what my 32V2 sounds like with an RE-20 and some external EQ,  as recorded on an SDR with an adequate amount of selectivity.

http://www.youtube.com/watch?v=1fqzfKsYqDs
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w6xr
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« Reply #5 on: November 04, 2009, 08:06:27 PM »

Thanks so much, now I have a lot to do.  The reality is the 32V3 is so heavy to move it even a little!

It's a good TX and I'm committe to make it better.


73,


Natan W6XR
Freeville,  NY
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« Reply #6 on: November 04, 2009, 08:20:53 PM »

There is an article in this month's Electric Radio by WA0SPM on building a battery powered vacuum tube mic preamp that allows  him to run his modern low Z dynamic mic (an Electrovoice RE27) with his 32V2.  It's on p. 8 of the October Electric Radio.  The thing with the vacuum tube and his choice of caps for rolling off his high and low frequencies may not be of interest to you but at some point the circuit may give you some idea of how to run a modern  low Z dynamic mic with your rig if you ever wish to do that.    If the xtal d104 works well you may not ever need to though.

73

Rob
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« Reply #7 on: November 05, 2009, 09:41:58 PM »

Some years ago I used to work a guy from time to time  (cant remember the call) who sounded VERY good on a stock 32V3. He said it was stock and he used a Sure 55s mic, which has a low med and high imp switch, dont remember what position it was in, maybe it was his voice, but the setup sounded fantastic.

When I tried it, it sounded like crap on every setting....

Brett

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« Reply #8 on: November 05, 2009, 10:41:04 PM »

Old desk mikes from FM base stations seem to work well. They also got a lever to lock the thing to TX.

I didn't mean to throw so much out there. Got carried away.
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