Preamp

Would you be surprised to learn that there is a real difference between volume controls and level controls? Unfortunately today, controls which are really “level” controls routinely get tagged with the label “Volume”. But what’s the difference?

Having performed the investigations I wanted on the gain stage on which I was working, I then moved on to another part of the project. And the chaos from a couple of days ago was nothing compared to yesterday.

For the last couple of days I’ve been revisiting an idea for a “tone control” preamp. I had originally decided to go one way with the design, but then something happened to change my mind. My ears got involved.

I was looking at some web page data this morning and I think I may have given a false impression in my post “You Never Really Know…” back in early November.

Whenever I make a decision to start a project for this website, I never really know how that particular project will be received. And sometimes I get surprised. This is definitely the case with the new Source Selection Preamp.

It really is amazing how much I can accomplish when suddenly I can’t work on all the projects which have been keeping me busy for the last month. The SSU assembly is complete and I’ve done some testing.

As it turns out, there is a crew working on painting the exterior of my house today. Since I can’t work on many of my outdoor projects around the house, I thought I’d take the opportunity to get a little done on the source selector unit.

It took a while but the DC checkout is complete and I’m finally ready to finish assembly on the Source Selector Unit.

The resistors for which I’ve been waiting showed up this afternoon. Time to get back to work on the Source Selector Unit.

As I said before, I decided perform the DC testing of the tube assembly for the source Selector Unit in the clamping fixture. But just as I started testing, I ran into a little problem.

I got some time today to finish out the vacuum tube assembly of the source selector unit.

I can’t believe I did that! How could I have been so thick headed to make this kind of mistake?

Yesterday I was bottling mead but today I got a chance to wire up the buffers and tie in the coupling capacitors. This completes installation of all the signal chain components. All that’s left on the assembly is the final power supply wiring.

Between regular chores, gardening, and yard work today I managed to steal a little time to make some progress on the latest project. The gain stages for the source selector unit are essentially complete and I’ll begin working on the buffers as soon as I can.

This morning after installing the selector switch and all its pigtails in the chassis, I decided to install the base plate. The chassis in now all in a single assembly.

So what does one do when needing to keep twelve signal paths clearly separated and easily identified in a chassis? Actually the answer is remarkably simple.

I’ve been waiting for some wire I needed for twisted pairs and a Fender style fuse holder. I really like these fuse holders. I find them sturdy, good looking, and reliable. I have some cheaper ones but I reserve those for brassboards, quick projects, test equipment, and the like. While I’ve been waiting for my deliveries, I’ve been populating the chassis.

This topic is just the gift that keeps on giving. I first introduced the 4S preamp with the volume control on the output almost 12 years ago. And yet there still seems to be a tremendous number of misconceptions about the concept and performance ramifications of volume controls on the output instead of the input. This time the misunderstandings surround an output buffer cathode follower.

William Shakespeare famously wrote in the play Romeo and Juliet,

What’s in a name? That which we call a rose
By any other name would smell as sweet.

So this is my quandary today. What should I “officially” name my latest project?

I’ve been catching up on all my spring chores and managed to steal a couple of hours in the shop this morning. The Source Selector Unit chassis is now all sanded. I also took the time to cut and fit the aluminum for the top and bottom plates. Now my attention will turn to getting all the holes laid out and drilled on the aluminum.

So I had another productive day working on the Source Selector Unit / Preamp chassis. The chassis has all the major woodworking complete and is ready for glue up.

I got some time today to make grain decisions and cut some basic joinery. It finally feels like I’m making progress.

After a difficult and focused three months or so, I am finally getting back to my normal routines. And that means I’m returning to work on my Source Selector Unit/Preamp project. Thank you to everyone who has had patience with my absence from the website.

Since I picked the wood for the Source Selector Unit/Preamp project yesterday, I decided to spent some time in the shop today getting the chassis pieces milled to thickness and cut to size.

This is just a quick post to let everyone know that I haven’t fallen off the face of the Earth. These last few weeks I have been dealing with some emergent health issues which have prevented me from getting out to my shop. But I have at least selected the wood for my Source Selector Unit/Preamp.

I’ve been taking it easy the last week and a half in the run up to Christmas. But today Front Panel Express came through with the front plate for the new project. Time to get back to work.

The design of Source Selector Unit/Preamp is coming along and I think I’m going to spring for a professionally made front panel. Today I spent some time with the design software from Front Panel Express and I think the ability to provide custom engravings will be worth it.

Over the last several days I’ve made some progress on the layout of the new project. This is about the point where it’s time to start looking at wood.

I just posted the project page for the 12AU7 “Color” Preamp which I completed a couple of months ago. Why don’t you go check it out.

This is a topic that rears its head periodically on vacuum tube forums and discussions. Just a couple of days ago I was reading a forum thread on preamps and someone threw out the suggestion of using a triode strapped 6V6 as a preamp tube. Now this is nothing new. I’ve been hearing this suggestion, on and off, for at least ten years. But using a triode strapped 6v6 (or any beam power tube) as a voltage amplifier is a poor idea which simply doesn’t hold up to technical scrutiny.

Now that the color preamp is done (knock wood), I’ve been trying it out on different amplifier and speaker combinations. But there is a little issue. Unknown to most people is that your brain can really only hold onto an unadulterated memory for a minute or so at the very longest. Most of the time you’ve got less than about 30 seconds. After this interval your brain starts throwing out what it considers to be extraneous information leaving only impressions. And this is a big issue when attempting to make subjective comparisons.

There are a variety of sayings, axioms, proverbs, or quips which could apply to me this afternoon. A few which come to mind include:

“How wayward the decrees of fate are” - Vanitas Vanitatum - William Makepeace Thackeray

or this one:

“Pride goeth before destruction, and an haughty spirit before a fall” - Proverbs 16:18

or just the simple axiom:

“It ain’t over till it’s over” - Yogi Berra

In any event, it would seem that “Final Chapter” or no, I have one more post concerning the Color Preamp.

So I decided today that I really needed to close the book on the color preamp. This means that I couldn’t just prototype a new buffer and leave it at that. The preamp needed an end-to-end test in it’s final design configuration. So that’s what I did. The results were even better than before.

Lots of work has been going on the last couple of days working on an alternate buffer design for the 12AU7 “Color” preamp. As readers should know, I was not particularly pleased with the performance of the previous buffer circuit. It was bottoming out at too low a voltage. Now however, I have a simple change which yields much better results.

This afternoon I completed the “color” preamp and tried it out. I am very happy with the results.

The “Color” preamp has been checked out on the test bench and everything is working per design. Bias points are good and the gain stage is operating exactly as designed. However, the buffer design I used was from a different project, and it operates differently in this project than the last one.

I had thought that the coupling capacitors I ordered on Tuesday would not arrive until this Saturday. However, the gods of DIY decided to smile down upon me and the capacitors showed up in today’s mail delivery. In my opinion, two days from order to delivery is exceptional. I spent some time this afternoon with a soldering iron in hand and the Color Preamp is now fully assembled.

I made significant progress today. Most of the signal stage wiring is done. I could finish it up in about half an hour, but there are two little blockages in my path; one due to a supplier problem, one due to me.

I’ve been working on the color preamp over the last few days as I’ve had time. My volume control and output capacitors are supposed to arrive today so it shouldn’t take too much longer to finish it up. However working inside the small chassis is a challenge.

Since I’m still waiting for appropriate weather to finish the 6AS7 SET chassis, I’ve been making progress on a small project for the aluminum chassis from the last post. I decided to put together the “Color” preamp from the post a few days ago. This project will make use of one of the Hammond discount power transformers about which I posted a couple of weeks ago. It’s coming along well, but VERY slowly.

For a while now, ever since the “Universal Preamp” was posted on the site, I’ve been getting steady requests for a good tube preamp to provide some color to solid state amplifiers. My initial answer to this problem was to design a good quality cathode follower for the Universal Preamp. There was one problem, the volume control.

As everyone who’s tried to buy one knows, power transformers for tube projects can be expensive. Even small transformers for small projects. But today I stumbled on a great deal for a small preamp power transformer made by Hammond.

Since I first introduced the concept for the “Universal Preamp” over ten years ago on the DIYAudioProjects forum, there has been almost constant discussion of my decision to place the volume control on the output of the preamp. I have been asked questions about it numerous times over the years. I have also had some “spirited discussions” with some who vehemently disagree with that design decision. Well, I thought it was high time to explain that decision once and for all, and to discuss exactly what this configuration means for preamp performance and output impedance.

So, I was sitting here on this rainy Sunday morning going through some amplifier designs and looking through my stash of tubes and various amplifier building supplies. While standing there looking at the shelves of vacuum tubes, transformers, and various piece-parts, my eyes drifted up to an amplifier sitting on the top shelf. It was my 6AS5 µ-power UL amplifier documented here. But why was the little amp sitting on a shelf collecting dust? Then I remembered.