Stupid question about transformers

[Erik Mandaville]

Very good, TR.

Let us know how things go as you continue. Consider the use of the single point ground I mentioned. Called a 'ground bus' or common ground, it, itself is not connected to the chassis. All ground connections go to it, and then A single wire connects the ground bus to someplace on the preamp chassis. This connection is usually made near the inputs to the preamp.

Are you using insulated RCA jacks for input and outputs? These use a shouldered plastic washer to keep the body of the jack from touching the metal of the chassis. If so,make sure you connect all the ground tabs on the jacks to the ground bus -- or whatever method you are using for the circuit ground.

Have fun,

Erik

[Erik Mandaville]

Ryan:

I think you hit the nail hard on the head with that analysis! I built this circuit 8 years ago, and tried to remember some of the things I encountered in the building process. I have since used some of the parts of the preamp for other projects (TR: This doesn't mean the preamp isn't worth anything -- it means as usual, I was working on something where I suddenly needed a part that happened to be in the preamp. It's a good preamp that will give you years of great service!)Anyway, I do remember scratching my head about that ground connection on the rectifier.

The heater supply is in essence a floating supply, and so should not be grounded to the primary circuit ground. (TR: If you did this as we both mentioned above, lift that ground) maintain the polarity of C6 (just don't ground it) as shown in the schematic, but don't chassis ground that one end of the diodes in the rectifier. Rozenblit is in fact trying to simulate a CCS for improved regulation etc, and the 95 VDC imposed on pin 9 is part of that design element. There is also some theory dealing with the relationship between plate resistance and amplification factor, which might be better left to personal reading and study than in this effort to help TR get his project working.

Although designed mainly to drive his OPTLs (output transformerless amplifiers), which have a high input impedance, he does make provision for using this preamp with amplifiers of lower input impedance/s -- this being the low output impedance (AC coupled) cathode follower. I have no idea what amplfiers this will be used with or the length of interconnect to connect the preamp to your amp/s, but you can try both outputs to see which you prefer.

TR: Please let us know when you see those tubes glow for the first time

BTW: What ARE you going to connect this preamp too?

Erik

[trvale]

I see the light! The tubes are glowing beautifully now. I can't stress how helpful both Ryan and Erik have been. I would have been banging my head against the wall and never have got this working without you two so thanks. I hate to be bothersome, but I have one more question. I think I have connected the POT incorrectly (another one of those things I just guessed on). It only works from about 11 o'clock on, counterclockwise. Going clock wise from 11, I get no music, just loud hum. The way I have it hooked up is input going to the left lug, center lug jumpered to the right lug, right lug is output. To fix the counterclockwise movement I know just flip the input and output, but this will give the same problem I have now. How do I get volume control from beginning to end?

Erik, I am currently testing this preamp on an Onkyo solid state HT reciever. I will be using this with my S-5 Electronics K12-M tube amplifier, but right now it is in pieces. I am in the middle of putting it into a chassis that matches the pre. It sounds great when I have the pot set pretty low. I think that it also has WAY too much gain for either the reciever or my tube amp. 23dB is over kill. From what I have read in the book, the cathode resistor will determine the amount of gain. R7 + R8 is the resistance on the cathode, which is 25.2k. If I wanted 10-12db of gain, what value would I want R7 and R8?

[Erik Mandaville]

I'm glad the tubes are glowing now. But test all your voltages again before you connect this up to your system. It would be good to know your plate voltages, the voltage on pin 9, and the voltage taken across pins four and 5 for your heater supply. Always do that sort of thing first before connecting to your system and applying a signal. I think Ryan would agree with this, too, TR.

Your volume control isn't hooked up right. The right solder lug isn't the output -- output is from the center lug, also called the wiper. And remove the jumper between the center and right lugs! One side of the control is grounded, the other side is the input of the signal that comes off the selector switch, and the middle lug is the output to the circuit -- the grid of the 12AU7, or pin 2 on the schematic. What lug is the input depends on how you have the control is oriented.

But are you using a stereo or two mono controls? The symbol for R10 is a variable resistor which is all a volume control is. If using a stereo control, you can jumper the two ground lugs on the control, and then attach a single ground from those two jumpered lugs and the circuit ground. Bot DO NOT just randomly jumper connections -- if you don't hear anything, you probably shorted your signal. If you are using a stereo control, one half of the control will be dedicated to one tube (you choose which one!), and the other half will be associated with the other tube. Connect input and wiper (center lug) connections accordingly. The only jumpered connection in this case should be between the two lugs that are the ground connection to the potentiometer (on a stereo control). YOu can also connect the two grounds on two mono controls, and then make one connection from that to the common circuit ground.

Gain: Don't mess around with cathode bypass capacitors and resistors now -- you can end up possibly creating some instability -- probably not, but maybe. There are other more straightforward ways to reduce the sensitivity of the preamp, but lets get some music coming through this thing (without too much hum or noise) first. If you find the volume controls overly sensitive, that's a pretty easy fix to make.

To wire your volume control:

With the preamp upside down, I'm assuming the controls are in the front of the chassis. Looking down at the controls, orient the solder lugs so they are face up. This is for a simple, carbon comp pot, by the way -- with 3 lugs.

1. Set your multimeter to ohms

2. turn the volume control all the way down (lowest volume position)

3. The input in this orientation will be the left solder lug

4. If you measure between this left lug and the center terminal, your ohm meter will read (about) 100kohms (on 100K pot). As you turn the shaft counter clockwise, the resistance will go down.

5. The ground lug is on the right. If you measure between that and the center wiper, you will get a reading much lower -- something like .1, .2, .4, 1.2 ohms, etc.

6. So the lug you want for the input is the left lug; the center one will connect to pin 2, and the outer right lug will connect to your ground.

Gotta go!

[trvale]

Erik - I checked the voltages, ~295v to the plate, 95v to pin 9, pin 4 and 5 about 100v. Is the 295v within acceptable error, or is it a problem? Now that I have wired the pot the way you explained, I get music throughout the whole movement of the pot. Only problem is that I have a lot of hum and noise, it gets quieter as I turn the volume of the preamp up, almost inaudible at full volume. It actually gets quieter not just drowned out by the music. I read that if I put the transformers at 90 degree angles of each other it will reduce hum, will that help in this case? Also the pot that I am using is a stereo radioshack model that has 4 lugs. Here is the info on it, 271-1732. It explains why I jumpered lug 2 to 3. The volume control is actually not very sensitive at all, it goes from no volume to max volume, without a gradual increase in volume. What is that 4th lug for?

The reason I am worried about gain is not because of the sensitivity. When I hook the preamp to a device with 2v (like my dvd-a player) output I get distortion at any volume. When I hook it up to my computer where I can vary the voltage output, same thing happens at 2v, but when I lower it down to about 1v or lower is when I can actually listen to music without distorrion. I think this has to do with the fact that my reciever can't handle huge voltage inputs like that. So by decreasing the gain by half, I should be able to use the standard 2v output that most devices use. Does that sound reasonable?

[Erik Mandaville]

Hi TR:

Well, since Radio Shack volume controls (from here onward we'll call these things 'pots')are pretty cheap compared to what's available try getting both a 10K ohm audio taper pot, as well as 100k ohm version. A good way to set up the volume adjustment and switching ability on this preamp, is to use two mono pots (one for right and one for left)on the right and left side of the front of the chassis, and the selector switch in the middle. If you now have a stereo volume control on one side and the selector switch on the other, you could re-locate the switch to a newly-drilled middle hole, and use the former switch hole for the other mono volume pot. Sorry! I know this sounds like lots of work, but it offers some advantages.

1. Tracking on inexpensive stereo carbon pots is not held to a very tight tolerance, meaning that the resistance between one and the other can sometimes be enough to perceive.

2. Also, a stereo control does not offer the added ability to regulate output enough to compensate for both differences in tube gain and poorer tracking characterisics.

With two mono pots, one sort of gets a built in balance control, and greater control over all of each channel. Moreover, they are just easier to work with.

If you are noticing that the preamp operates more favorably in terms of gain when using the variable output of the preamp, the use of a 10K pot vs 100k pot could very well be helpful. 10K is less sensitive than 100k (I know that sounds backwards, but that's how it works!), and might work well.

So, what I would do, is buy a total of 4 pots -- two 10k, and two 100k. These would be just standard, 3-lug controls, log (or audio taper). Don't get a linear taper pot, since they tend to be very sensitive at lower rotations. They go sort of straight up (hence linear)from quiet to very loud. A log or audio taper is more gradual, and the rotation of the shaft approximates the increase in volume. If you can only get two mono pots for whatever reason, get two 100k ohm. Or just find a decent stereo control, minus the loudness tap. On a 3 lug terminal, don't jumper anything but the two grounded ends.

Ok, another thing: is the volume by any chance isolated from the chassis? For example, did you paint before you installed the parts? If so, your volume control may not be getting adequate ground -- which can cause noise and hum. It's good practice to very neatly scrape away (an x-acto knife works well)the paint in only the area where the body of the pot makes contact with the chassis. Also remove any paint that may be in the area of the point at which the circuit ground is connected to the chassis.

Consider the two mono pot idea, TR. It offers lots of advantages over an inexpensive stereo control. But alas, it requires work. A friend of mine reminded me that building for oneself is not always an easy thing to do -- and he is right about that. I have worked for literally months trying to hunt down sources of noise.

Make sure you do not have signal carrying wires bundled together with high voltage plate supplies or close to AC (not your airconditioner!)

At least you have got music coming through now -- outstanding work! But there may be a bit of a journey yet before things sound acceptably good. Sources of noise and hum can be elusive, and can take real effort to find and make the needed repair.

Please consider to mono controls on either side, with the selector switch in the middle. Are you using shielded cable from the inside of the RCA jacks over to the selector switch? If not, you should at least twist signal and ground leads to help increase immunity to RFI and EMI. MAKE SURE ALL GROUND CONNECTIONS ARE MECHANICALLY SOUND AND CLEANLY SOLDERED.

Ok...I'm tired, TR. Great work, let me know how it goes. Email me anytime you would like, and I'll try to work with you on this.

Erik

[Erik Mandaville]

...one other thing:

If you end up using two 100k pots, you can lower the strength of the input into the preamp with series resistance. This effectively accomplishes the same thing as lowering the output of your CD player with the variable output, but do use the FIXED output -- this is much more desireable than the other.

If you relocate your selector switch to between the two volume controls, it might be near enough to the following:

Instead of using a wire to connect the two inner poles of the selector switch over to the inputs on the volume controls, replace those two wires (one for each channel) with a 100k resistor -- which is the same value of the pot you are using. I can't remember the math off hand on this (maybe I never knew it!) but it will decrease the gain by a few dBs. If it's too much, you can lower the value, if not enough, you can increase it.

We are only touching the surface, TR! But you are close, now!

Erik

[Erik Mandaville]

And another thing!

B+ of 295 is not too bad. It's maybe a little on the high side copared to spec, but not too bad. Other things look ok. Lets not worry about that now, since it's pretty easy to take care of.

TR: Are you set up to either email me or post a picture of the inside of your preamp? If you don't want to do this for whatever reason I understand. You should have seen the first amp I built. I proudly showed my dad (who knows a bit about all of this), and he burst out laughing! I promise I would never do such a thing -- I just want to help you get this project going. Because once it's done, you'll already be thinking about the next one. Trust me!