Another thread on preamp 'gain'

[mikebse2a3]

pg795

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pg796

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Last pg797

[Erik Mandaville]

Mike:

I hope Jeff Lessard doesn't mind me quoting him, and for this I apologize in advance if I miss it by a word or too -- actually the phrase goes back probably to biblical times: "There is nothing new under the Sun."

Honestly, it's great to see this information, and I have learned so much from reading now-antique literature, which without a doubt still applies to a very significant degree. So much of all of what we now experiment with, design, and hopefully enjoy, has its roots in soil planted with the same seeds for generations. Yikes! this is starting to sound profound, but it's true, I think.

I've been recently doing some reading on digital amplification that's been equally provocative, and has made me take another glance at the now unused Teac, sitting idle on the floor like some sort of ultra-modern pet with its power-cord-tail wrapped in neat loop around it. It's up for adoption, and a new owner already in mind. On paper, the thing is nearly perfect -- yet so much so that I want and need to have something more to........

...sorry, this is getting entirely away from the subject of this thread.

Thanks for posting that Radiotron information,

Erik

[JulieHeartKlipsch]

I can't say I am following any of these fancy words and diagrams. [:$]

Does it matter to non-engineers who are just listening to music?

[][]

J.

[Erik Mandaville]

Julie:

I know, this is kind of crazy. Enjoying music is THE single most important aspect of this; and as often as I tell myself that, I continue to tie myself in knots with resistors, capacitors, volume control snarls, etc.,

You are right.

Erik

[mikebse2a3]

Mike:

I hope Jeff Lessard doesn't mind me quoting him, and for this I apologize in advance if I miss it by a word or too -- actually the phrase goes back probably to biblical times: "There is nothing new under the Sun."

Honestly, it's great to see this information, and I have learned so much from reading now-antique literature, which without a doubt still applies to a very significant degree. So much of all of what we now experiment with, design, and hopefully enjoy, has its roots in soil planted with the same seeds for generations. Yikes! this is starting to sound profound, but it's true, I think.

I've been recently doing some reading on digital amplification that's been equally provocative, and has made me take another glance at the now unused Teac, sitting idle on the floor like some sort of ultra-modern pet with its power-cord-tail wrapped in neat loop around it. It's up for adoption, and a new owner already in mind. On paper, the thing is nearly perfect -- yet so much so that I want and need to have something more to........

...sorry, this is getting entirely away from the subject of this thread.

Thanks for posting that Radiotron information,

Erik

Erik you are very welcome.

The "Radiotron Designers Handbook by F. Langford-Smith" is a wealth of information for anyone interested in electronics and tubes especially.

An experience I had once about the important role of paying attention to impedance issues was when I built a very high quality passive pre-amp using 50Kstereo volume controls with both sections paralleled to create a 25K control for each channel. I was using a CD Player with a Tube D/A Converter into the passive when I picked up a Creek OBH 10 Remote Passive Volume Control. The Creek Control's Values where 50K and the sound was much better in the bass and clarity was better than with my Homebuilt Unit. I decided the Tube D/A Converter must not like the 25K Load of my Homebuilt Unit so I rewired it as a 50K Control and sure enough the sound was every bit the equal of the Creek. This is one reason I would advise not using low impedance attenuators if possible especially if you might want to use Tube Equipment with it. With Tube Equipment a range of 50K to 250K is often used with 100K being a good overall compromise.

mike[]

[Erik Mandaville]

Mike:

Again, some very helpful information there. You know, I've paralleled the two sections of a stereo control on several occasions -- as you, almost always in relation to a passive control/switchbox -- and wondered about the resulting tracking behavior that resulted from that paralleled connection. Since parallel combinations of fixed resistors is so common, there is no reason -- at least in theory -- that one couldn't do the same with variable resistances, but it did make me wonder about what was actually happening to the signal, especially when using less expensive carbon-comp potentiometers.

As far as passives go, which I think can work wonderfully well in the right circumstances (i.e. suitable input/output impedance relationships), my feeling is that an ideal way to go about it would be to use transformer coupling -- where an input transformer presents an appropriately high input load, but a very low, easy-to-drive imped. on the output side. With the usual passive attenuators, it seems that the position of the volume control is going to play an important role in terms of output imped., and that even a 50K ohm potentiometer, depending of course on the circuit it's used in, might bring about a less than desirable imped. (overly high) into the next stage; which or course would be compounded by the capacitive load of interconnects if they are long. Have you opened that Creek up? I've always been curious about what is in them, and what the circuit actually consists of.

the usual 'edit': Mike, let me rephrase something -- I didn't mean that a 50K pot wouldn't work, which is how it sounds above. I don't mean that, and as you said you have gotten great results with that. My thought was that the value of the control also plays a role on what happens on the output side into the next stage -- the amp. I agree with you, and this is what Mark was also saying, that higher imped. on the input is a 'good' thing, but if one were using a large value potentiometer to obtain that high value, what comes out the opposite end and travels onward to the amp must see a proportionately correct load for best coupling between the two. I suspect this is why I have seen on numerous occasions references to using a 10K ohm control in a passive -- which is said to have, and I have experienced myself -- a reasonably high input impedance (thought more would certainly be better), but lower on the output end. My interconnects at the time were literally no longer than 10 inches. I bought everything from Radio Shack for next to nothing, and used my system this way for quite awhile.

The compromises with all of this seem endless.

I have a 300B amp to finish so we can listen to it this afternoon -- I'm excited about it! More later,

Erik

[sfogg]

"I have a 300B amp to finish so we can listen to it this afternoon -- I'm excited about it!"

Cool, when you get a chance I'd love to hear details about it!

Shawn

[Erik Mandaville]

Shawn:

As I mentioned, The filament supply is what has become the hurdle. The Moth 2A3 used regulated 2.5VDC, but I need twice that for the 300B. I removed the regulators, and installed bridge rectifiers plus filter section using the 5 volt windings (which were a bit lower than 5 volts to begin with) that were used for the 2A3 regulators. However, under load, I was getting almost 2 volts too low. Not good.

The Moth is not an amp that I want to drill full of holes for experimentation purposes, and I had given up the plan yesterday. Last night I thought of a way I could use a 6.3V (3A) filament transformer I have, and just drop that with about an ohm for what I need. This implies that I would only use one of the 5 volt windings on the stock transformer. This would reduce the current draw on the stock transformer (not much, but a little), and I do have space in the chassis for one extra filament transformer.

We might be listening to 300Bs later this morning. Last night I put the Moondogs back in the system as the main L/R amplifiers, and they sounded really good. They really are an excellent pair of 2A3 amplifiers.

But, I did mention to you that, just for fun ('curiosity' is probably a better word), I turned the volume up on the Lexicon to see what it would take to drive them very briefly into the clipping range. It didn't take much, actually, although the volume was quite a bit louder than what we normally listen to.

Lastly: I need to offer a very sincere thanks to Jim T. for such an amazing and unexpected gift -- a pair of 300Bs in perfect condition! I can't thank you enough, Jim, they are really awesome tubes! This whole thing would not have been possible without your generosity and kindness. If it turns out I'm going to have to modify the chassis of the Moth more than I would want for this project, I'll stop and rebuild a completely new design in two other chassis for a pair of 300B monoblocks. The Moth will be returned to its former 2A3 self.

Erik

[JulieHeartKlipsch]

I just want to say that I am NOT against engineering and technical

things at all but just can not follow what was being said and wondered

if there is any easy summary of what this means to an ordinary person

listening to music. "Real Life Application" in other words.

Thanks,

J.

[dragonfyr]

While I have the hardcover "Radiotron Designers Handbook", has anyone looked at the CD version? And if so, how searchable is it?

They have the potential to be either fabulous or a mess ... so I am quite curious!

[mikebse2a3]

While I have the hardcover "Radiotron Designers Handbook", has anyone looked at the CD version? And if so, how searchable is it?

They have the potential to be either fabulous or a mess ... so I am quite curious!

I'm not sure what all CD versions are avaible at the present time.

I bought mine from AUDIO AMATEUR PRESS Order no: CDRDH many years ago.

In The Version I have you can jump to a chapter directly from the table of contents and then link directly to each topic. You then can scroll through the appropriate pages of the Topic.

I hope this description helps.

mike[]

[mikebse2a3]

I just want to say that I am NOT against engineering and technical things at all but just can not follow what was being said and wondered if there is any easy summary of what this means to an ordinary person listening to music. "Real Life Application" in other words.

Thanks,

J.

Good Question Julie

What happens to a system using Very Efficient Speakers like the KHorn/LaScala/ Belle Klipsch is because they are 104db efficient versus 85db to 90 db efficiency of the average speakers on the market. With this 15db to 20db (This is often the amont of Gain that a preamp gives to line level sources(CD Players for example) and is also why we can sometimes get by without a pre-amp with our KHorns Systems) increase in efficiency not as much gain is needed to reach normal listening levels. So what happens is the Volume Control ends up only being usable toward its lowest settings where they often have the largest increases from one notch setting to the next setting which makes it hard to set the Volume Control just where we want to listen at.

Also if the electronics hum/noise floor is high it will be very noticible on a very efficient speaker like a KHorn.

I believe if we need to use an attenuator the best place to use an attenuator for most systems would probably be on the input of the Amplifier because when placed there it will also attenuate any inherent noise that the pre-amp has and thus should give us the least hum/noise in our system.

Put Simply High Efficiency Speakers aren't the normal speakers people use and so most systems are put together with the thought of these 85db to 90db speakers. This is also why alot of Loudness Controls systems don't work good with very efficient speakers because they aren't variable based on our speaker's efficiency.

mike[]

[mikebse2a3]

Erik asked:

Have you opened that Creek up? I've always been curious about what is in them, and what the circuit actually consists of.

Well to be honest I never bothered to open it up. I knew it handled the sound very good so I left it alone. Anyway since you asked I thought why not see how its made so here are a couple of pictures.

The control isn't very impressive but only a few active pre-amps have sounded better in my system.

mike[]

Edit: Notice the board says OBH12 which is a unit that has multiple inputs so evidently they use the same board in the OBH10 and just leave out some componets.

[mikebse2a3]

Creek OBH10 control picture

mike[]

[DrWho]

I'm a bit late to the party, but thought I'd post two somewhat relevant links that I found when looking up "impedance matching"...thought you geeks might find them interesing:

http://www.prosoundweb.com/studyhall/rane/unity.php

http://www.prosoundweb.com/studyhall/lastudyhall/gain.html

[Marvel]

JulieHKlipsch,

If you were asking about Erik changing output tubes... Tubes have two

different voltages connected to them. What Erik is talking about are

the heaters. For a tube to work, the cathode is heated up by running a

low voltage through it. As it heats up it will allow the electrons to

boil off the surface. This is part of what makes tubes work. Those

electrons are then attracted to the anode because of the difference of

potential between them. A screen or grid is placed between the cathode

and anode. A varying voltage there (your input signal) varies how many

electrons get from the cathode to anode. The voltage between cathode

and anode is higher than the grid, so you end up with a small signal

being "amplified", as the voltage or signal goes up and down on the

grid, the higher voltage between the cathode and anode goes up and

down. This is where the term "valve" comes from when applied to tubes.

Different tube types use different heater voltages, but the common

voltages are 12, 6.3 , 5 and 2.5 volts. You usually get that from a

small tranformer or seperate winding on the main power transformer.

Since Erik was replacing a tube with one that requires a different

heater voltage, he had to get the proper voltage from somewhere else.

That is it in a nutshell, and I'm sure that those more knowledgeable will correct me if I am wrong.

There are other things going on as well. The phase changes, etc. but we won't go into that.

Bruce

[Erik Mandaville]

Marvel:

Wow, what and excellent explanation your provided! Thanks for sharing that.

Erik

[Erik Mandaville]

Mike:

I do appreciate your going to the trouble to take those pictures of the Creek. It's more complex than I thought, and for a minute thought the body of the VC was connected to small round transformer. Is it part of the motor for a remote volume control, or something?

Also, I wonder if this little preamp, while passive, still incorporates a small amount of EQ circuitry, which would help offset potential HF losses related to lower volume control rotations. If this were the case, it would be similar to what guitar amp techs sometimes did/do on older muscial instrument amps. This is as simple as a small bypass capacitor between the input of the VC and wiper, and acts as a high-pass filter when the volume is turned down. It's supposed to compensate for HF losses at lower volume. The cap is a small silver mica, in the range of something like 47-100 pfd. I tried it, and it really works.

It looks like a well done control!

Erik