High-Gain Amplifier Circuit

[PRR]

> what should I be looking for in a transformer?

Line output, 600Ω on one side, 2K to 5K on the other side, +20dBm to +28dBm, no DC.

This is no longer a common part.

If you can find a high quality 150Ω/600Ω:150Ω/600Ω +28dBm transformer, that may work. Connect it as 600Ω:150Ω and call the secondary "150Ω", the primary will be 2,400Ω, near enuff.

 

[NewYorkDave]

Call Edcor and ask for a WSM2.4K/600 (quick-disconnects) or EM2.4K/600 (wire leads). They should cost less than $20 each.

As PRR says, you can also use a qood quality 600:150 transformer. The primary inductance of some models is high enough to apply them as 2.4K:600 transformers. Even an inexpensive Edcor 600:150 model (one of the $5 +24dBM PC-mount models) tests out at about 15H of primary inductance, which will work).

AS for the interstage transformer, 90K:10K is higher than I expected! I agree that that transformer will most likely have too high of a winding resistance, certainly way higher than what the reflected impedance is supposed to be (2400 ohms). The winding resistance should not be more than ten times the driving source impedance. The source impedance of the WCF in my circuit is about 75 ohms.

 

[idylldon]

[quote author="NewYorkDave"]Call Edcor and ask for a WSM2.4K/600 (quick-disconnects) or EM2.4K/600 (wire leads). They should cost less than $20 each.[/quote]

Just talked to Edcor and they don't have any such beast. :sad: I'm all finished with the exception of the OT and that 10uf cap feeding the OT. The largest poly cap I have at the correct voltage rating is 4.7uf right now.

My voltages are a bit low (down about 30) because of the transfomer I used, but I have a few more lying around and perhaps while I'm waiting for the other parts I'll check to see if I have one with higher secondary voltage. All the voltages are in line, though, so it's just a matter of getting some more juice at the beginning of the circuit.

supposed to be (2400 ohms). The winding resistance should not be more than ten times the driving source impedance. The source impedance of the WCF in my circuit is about 75 ohms.

The DC resistance of the Stancor is 515 ohm primary and 1.9K secondary, but there's obviously much more to take into account here.

Cheers,
--
Don

 

[NewYorkDave]

For the output coupling cap, you could use a 'lytic or two 4.7uF film caps in parallel...

 

[rafafredd]

I think that if you are using a 2:1 step down at the output, just one 4.7uF cap will be enought. But if you are using a 600:600 1:1 transformer, I think you would want at least 22uF, but 47uF or even higher would be better...

 

[NewYorkDave]

A little simple math is just as easy as guessing.

Remember that the primary inductance is in shunt with the reflected impedance. So it's not quite as simple as calculating the rolloff of 4.7uF coupling to 2400 ohms. As a simple rule of thumb, for flattest response, put your (calculated) -3dB point an octave below the lowest frequency you want to amplify. Even ignoring the shunting effect of the primary inductance, 4.7uF/2400 ohms gives -3dB at 14Hz, whereas 10uF/2400 ohms gives -3dB at 7Hz, which is better. Unless the primary inductance is very high, the actual rolloff point is going to be at a higher frequency than this simple calculation predicts, so better to leave some margin.

 

[idylldon]

[quote author="NewYorkDave"]Call Edcor and ask for a WSM2.4K/600 (quick-disconnects) or EM2.4K/600 (wire leads). They should cost less than $20 each.[/quote]

Where did you get these part numbers? The Edcor folks drew a blank when I called the.

So, does anyone have a suitable tranny they want to sell?

As PRR says, you can also use a qood quality 600:150 transformer. The primary inductance of some models is high enough to apply them as 2.4K:600 transformers. Even an inexpensive Edcor 600:150 model (one of the $5 +24dBM PC-mount models) tests out at about 15H of primary inductance, which will work).

I'll visit their website again.

Thanks,
--
Don

 

[PRR]

>> Call Edcor and ask for a WSM2.4K/600

> Where did you get these part numbers?

WSM2.4K/600 would be a special-order. Dave says Edcor is pretty good at that: setup charge is small and they get the order right (the second time).

Or just get a standard WSM 600/10k. Wire it backward, and use just half the 10K winding to get 2.5K. Using half a winding gives a small added loss, maybe 1dB, not important.

This core is rated 0.5 Watts 20Hz-30KHz. That computes to +27dBm. Dave's plan, perfectly balanced, should hit +30dBm at gross clipping. He claims +26dBm at low distortion, with balance typical of real parts. So a +27dBm core is "just enough". If we were building a console that cost $25,000 in 1958, we might use something bigger just to keep tested THD low below 50Hz. But hey: this is $9 iron that does the job fine.

BTW: they sell the same core with a 5 Watt rating 40Hz-30KHz. Power rating is mostly about how low in the bass you will test. Since you probably don't have any 20Hz instruments in your studio, that 0.5W rating is conservative.

 

[NewYorkDave]

It so happens that the circuit was breadboarded with an Edcor WSM10K/600 with only half the primary used. Performance at very low frequencies (~20Hz) isn't as good as it could be because of only half the primary being used. I expect better performance when I put a true 2400:600 transformer in there.

The EM2.4K/600 is a custom job I ordered. The original ones they sent me had the wrong turns ratio. They 'fessed up to the mistake and I'm awaiting the replacements. The holdup is due to the fact that they had to special-order smaller wire and different bobbins to re-do the other custom I had ordered (10K:100K). They had gotten the ratio correct on the latter transformer, but the primary inductance tested too low for my liking.

 

[rafafredd]

WOW! Edcor will do 10k/100k for you? Is it for the input?

 

[idylldon]

Since Dave posted this schematic I've been tinkering around with it, and I'm pretty happy with what I'm hearing. He's been most helpful as I've worked through some bugs (of my doing). It works extremely well as a DI (my Strat sounds like a Strat through it--crisp and clean), and I've been trying to get a few more tubes to try to get the noise floor a bit lower when using a mic. Since the 12AV7 tubes are so cheap, I plan to get a few and screen them for low noise.

I didn't get an output tranny yet, so I've just used a 10uF cap to connect to the output. Seems to work just fine this way.

Anyway, here's a pic of it. It's built on an old Webster Electric mic pre chassis. I don't have the rackmount faceplate on it yet because it requires some cosmetic surgery and it's too cold to paint anything up here right now.

NYDave's Pre

Cheers,
--
Don

 

[NewYorkDave]

Based on some experiments I did during my Christmas vacation, and Don's observations about the unit he built, here's an updated version.

Page 1
Page 2

 

[Bauman]

thanks!
:thumb:

 

[daArry]

x2! :guinness: :thumb:

 

[rafafredd]

Fantastic. The variable gain at the first stage makes this an even greater tool!

 

[MikoKensington]

Thanks, again.

 

[NewYorkDave]

Thanks.

I was worried that reducing the feedback resistor around the first amp block would load down the plate of V1B too much; and while it does reduce maximum output at the lowest gain settings, it turned out to be not much of a problem in real life. The undistorted output of V1B is always more than enough to drive the second amp block to its maximum output.

Since varying the value of the feedback resistor changes the load on the plate of V1B and thus the open-loop gain of the V1A/V1B block, I found it much easier to find the feedback resistor values by experimentation rather than by calculation. Then I selected the nearest 5% standard values. Even with this rounding off, each step is within a couple tenths of a dB of its nominal value.

The unusual gain control arrangement, with selected resistors shunting across a fixed feedback resistor, was to ensure that the amp never goes open-loop, even if the switch becomes intermittent.

With a 1:10 input transformer and a 2:1 output transformer, maximum gain is 70dB, which should be more than adequate for just about any mic. You may have to select V1 for lowest noise for best results at the highest gain settings. Metal film grid and plate resistors would be a good idea, too, but watch the voltage rating on those plate resistors!

 

[Consul]

I'm definitely interested in this one! Thank you much, Mr. New York Dave. :thumb: