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Opinions on integrated amp project

As I begin to put a list of parts together for a guitar amp project it occurred to me that I might save a little on shipping if I ordered some of the parts for the next project at the same time. The problem is that I am a little fuzzy on the details of how to best accomplish what I want to do. :xeye: :cannotbe:

I have a biamped system and had considered different options of building the separate power amps with built in crossovers or a preamp with crossovers and straight forward power amps. On third thought however I thought that a complete integrated system could save on PS costs and most likely still be adequate for my needs. The layout is full(wide) range 5.24" drivers on top and 10" woofers on the bottom all OB.

The WR is quite sensitive (at very loud levels the "power meters" on my Vintage Sony receiver barely touch 1 watt) so I am planning SE there and PP for the woofer amp. I will probably use triode strapped EL34s in both cases. I will have preamp out, and sub out as well as the speaker outputs. The difficulty comes in getting the speaker outputs and low level outputs in phase without excess VAS stages. Of course I guess there is no reason that the speaker connections could not be reversed is there? Does the output transformer secondary get connected to ground usually? If so does the transformer care which end is grounded?

My initial thought was to use a LTP as the input stage feeding one side to the SE amp and the other to the sub/pre out buffers and then using both sides to make it do double duty as the PI. But on further reflection I don't like the idea of having to make up perfectly balanced PP filters between that input stage and the PP output stage. So I would definitely want to take only a single phase through a filter network and then have a separate PI just before the output.

Well at that point the option of a simple CC input stage is added to the mix. I wonder if there would still be some advantage to using separate sides of a LTP to drive the low level outputs.

I have attached an overview of what I am envisioning. I would appreciate any input on how you would proceed.

BTW the WR would be run pretty much full range with a switchable LP filter as an option to save on power in the SE stage. Shown is a first order (switch out the one cap to put in 180Hz HP).

In the woofer section a HP at 50Hz would be combined with a pole at 50Hz for baffle compensation and two more poles at about 180Hz for a final 3rd order slope.

mike
 

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No opinions at all? Is my general concept that far off base as to make comments pointless? Was I too verbose?

Let's talk input tubes for a minute. I have noted that a lot of classic designs used 12AX7 for VAS or PI use yet I have seen many negative comments on these for just that application. Maybe their high gain was considered an advantage in those amps with GNF. Since I don't plan on using GNF would 6SN7, 6SL7, 6922, or 6N1P make more sense.

Maybe I will go ahead and start with a PP with built in crossover and subout as that is the most critical need right now.

mike
 
Mike,

Let me start with, sometimes less is MORE.

I am going to make an assumption that you are newish to tubes, given the post? Correct? And in that case I would say keep it simple, to increase your chance of success.

You said, you only hit 1watt peaks on your current amp. If you are wanting to do SE, why the separate amp for the woofer? A good SE amp could drive both easily at these levels. And if you truly are going to biamp to improve the low end, why tubes instead of solid state? Those bottom couple of octaves are the only place I'd say SS can give tubes a real run for their money, so if the idea is to truly have the best sound, I'd have to at least suggest you consider SS for the low end.

Are you using tubes now at all? If not, I'd definitely say, stay simple, build an integrated SE amp and skip the bi-amp and go from there. A good SE amp will trounce any solid state sony or the like you are using now, bi amped or not. Give us some more info to go on...
 
Hi Cycline3,

Yes I am new to tubes. I guess that I should have been more specific about where I am currently and where I am headed. I am currently biamped with separate powered sub (bi and a half amped?);) The speaker system is a DIY OB with pretty efficient 5.25" FR (old radioshack units that look like fostex clones) on top and a not very efficient (89dB as I recall) 10" Goldwood woofer. The Q of the woofer is over 1 and the fs around 40. The subwoofer is a 12" in a sealed enclosure of about 5 cubic feet powered by a salvaged plate amp out of an AR 8" powered sub. The plate amp was modified to improve the low end cutoff as the original was designed to protect the tiny woofer in a BR cabinet.

Currently I am driving the FR with the output of a vintage 55W/ch Sony SS receiver. Since it has no preamp outs I am feeding the headphone output to the aux input of an old Realistic 35W/ch receiver on which I have the bass turned all the way up and the treble all the way down. Obviously this is not optimal and a real crossover would be more appropriate. However given the baffle rolloff and the highish fs of the FR driver it integrates a lot better than one might expect. The FR starts to roll off at around 180Hz and the bass control on the woofer amp is 100Hz shelving control so it lines up pretty well. Depending on the situation I might also have the loudness compensation on in the bass amp. The Sub starts rolling in about 70Hz so there is some overlap there.

As it is this system has the best detail, balance, and imaging of any system that I have previously used (previously all box speakers) but I think that better control of the range of frequencies produced by the individual drivers will probably clean that up even more.

Going with a passive crossover and single amp is possible but I really don't like some of the compromises that I will have to make and most especially having to pad down the FR. That loss of efficiency I fear would have a negative impact on the fabulous dynamics that I now enjoy. Also given the condition of the old FR drivers I expect that they will be replaced sometime in the near future which would make any passive crossover obsolete.

So the biggest things that put me on to building new amplification are that even after cleaning pots and switches I have periodic hash and crackles on the bass amp (30 years old at least), I want proper crossovers to eliminate some of the excessive overlap (woofer plays up pretty high at reduced levels), and I want to play with tubes.

My plan is to cross the woofers on the bottom end at about 50Hz to protect the OB woofer from over excursion on movies, heavy organ, and synth music. I believe that is low enough to get most of the desirable OB bass effect on things like tympani but high enough to not stress the driver.

So at least initially the lowest two octaves will be covered by a sand amp (some day a transmitter tube PP with custom low bass transformer driving IB subs may be in the cards but not for now).:D

Since the current 35w sand amp has plenty of output and the frequency range needed is modest in the extreme it seems that a PP EL34 amp ought to work fine for the woofers I think.

You are probably right about not biting off too much at once but if I limit it to one amp the woofer amp with appropriate crossover is probably the most necessary item. I could build the woofer amp with built in active crossovers in the input stage (switchable) and a non-inverting buffered output to feed to the sub and drive the woofer amp from the headphone output like I am currently doing. A low pass ala Steve Bench on the buffered output would help with integrating the powered sub which currently runs up a little bit higher than I would like.

Eventually I will probably use the Sony only for tuner and phono preamp by feeding the record out to a tube preamp to drive all of the power amps. One step at a time makes sense as long as I keep the end game in mind.

Does this help you to understand my situation better?

mike

P.S. thanks again
 
mashaffer said:
Does this help you to understand my situation better?

Yes it does.

I feel fairly confident in saying that any tube amp, SE or PP, and a single one at that, would give you a much better soundstage and realistic reproduction than what you have now.

Do you have any friends with tube gear that would be willing to bring it over and try out? That would be a great place to start, by simply hearing some tube gear at your place.

For reference, I personally am NOT the SE, full range driver kind of guy. I am currently using a 40W PP amp driving B&W 703s with a small B&W subwoofer crossed over at 45hz. That's PP and 3ways with a sub, pretty much the opposite side of the scale.

Still, i do know that the SE/FR purists are trying to remove as many electronic components from the system as possible, namely and most importantly the crossovers.

So, it seems to me, that by wanting to use crossovers, you are defeating the point of the full range system to begin with.

I would heartily recommend you build a tube amp for your primary speakers, be it a small single ended or push pull. If you are going to use a subwoofer, then I would consider scrapping the goldwood woofers altogether. From the sound of it, you don't need massive sound output, and what you gain by using less parts, less speakers will more than make up for it. That means run your FR speakers at full range and supplement them with your subwoofer only.

Now, as for the sub woofer, use the dedicated crossover in it, or consider purchasing an amp for it with the built in crossover. They start as little as $40 (ive see on sale for as little as $25) or so at Parts Express...

That way, you do not have to worry about crossovers at all in your amp. You would only need to add 1 tube to your amp design - a preamp tube like 12au7 or 12ax7. Thats a twin triode tube, you would use each triode as a cathode follower to power your sub woofer out RCA jacks.... simply take the output from your first line stage amp and put that to your phase splitter/driver AND the subwoofer cathode follower. You then have a couple volts more or less output to drive your subwoofer amp, which should be more than enough for any sub.

Again, I have no doubt a tube amp will best your current solid state stuff, so start simple. If at some point you decide to add another tube amp ,that's fine... consider it phase 2. Still, the more complexity you add, the harder it is to get it right. So, again, less is more.

I should also say, if you haven't read the safety links, now would be a good time to do it.
Tubelab has a great safety page here too.
 
The situation may have changed drastically today. I picked up an Onkyo TX-SV525 HT rcvr that was supposedly not working. When I am listening to it play right now and it seems functional at least in the FM and preamp. I am listening through crappy speakers initially in case there is a speaker killing failure coming. If all is well and I can use it for the tuner and phono then I will be able to free up the Sony that I am currently using as the main amp for use on the woofers. In that case I would be able to go ahead with the fullrange amp as you suggested.

My subwoofer amp does have a built in LP filter and that is how I am using it right now but I would like to roll it off a bit lower than the built in is capable of but that can be done with a single triode per channel and a few passive components.

Thanks again for the input.

mike
 

jnb

Member
2006-12-30 11:55 pm
What you want is reasonable. It is all a big step and I would suggest you break it into small pieces, e.g. just build a power amp for now, or the pre, leaving room for more components such as crossovers.

The first thing you should do is make a roadmap. Measure the gain you are using from each amp to help you choose amps to build. Make a block diagram.

I feel you could approach it building straight power amps and putting the crossovers in with your pre. I would not worry about PI stages, swapping the speaker leads would do it for me.

Dont assume you'll need PP for the 10", just because we're talking about low frequencies. It might be fair enough to guess that your woofer would bottom out on an open baffle before using 50W. You may only need a small PP or large SE triode or a class A solid state amp. Can you use your amp with power meters to find out?

Does the output transformer secondary get connected to ground usually?
Typically.
If so does the transformer care which end is grounded?
Simple answer, no.

Study existing power supplies and try to see this for yourself. I think it's important that you understand their function.
 
jnb said:
If so does the transformer care which end is grounded?

Simple answer, no.

The transformer doesn't care, as long as you ground the same end of the secondary in your left and right channel to keep them in phase. Your feedback loop may care, you will oscillate if you ground the wrong end, in that case, simply flip it to the other end of the secondary, again, on both channels.
 

jnb

Member
2006-12-30 11:55 pm
Excellent choice. IMO building a SET amp would be the simplest and most rewarding first valve project.

Exactly how much power you need is one important piece of knowledge before you start such a project as there are many good output devices to choose from if you don't need much power.
 
What you are suggesting John is an interesting option. Where I live that would mean an ebay purchase for sure. Another option that I was thinking of could be an economical option though.

I have an old radio shack "mic mixer" that I haven't seriously used in quite some time that could be a chassis donor saving about half a C note. The box is (from memory) about 1.5 or 2" high by maybe 4" deep by 7 or 8" wide. It has 5 pots, a power switch and a red led on the front panel; 3 pairs of rca jacks and a couple of 1/4 jacks on the back along with two spst slide switches and the line chord.

Was thinking of using it for a small integrated with preouts. I would drill holes in the bottom for cooling air inlet, arrange bottles and output transformers on top and use a remote power supply (built on a chassis donated by a solid state Organ). I have to draw it out carefully to see but I think it will fit if I use 9 pin tubes like 6n1p, 6922 etc for preamp and these little edcore output transformers driven by EL84 or 6V6. I know the speaker impedance will be 8 ohms so that lack of other taps is not a deal breaker.

http://www.edcorusa.com/products/transformers/xse/index.html

The wiring diagram doesn't show UL connections but the specifications seem to imply that it does have them. The thought is that rather than using big bottles when I probably will rarely (never) need more than a couple of watts in the full ranges I could wire it switchable between triode and UL mode. From what I have heard these units are rather nice for the price. Response should be more than adequate for a 2-4W amp designed to run only above 100Hz.

I figure adding some more RCA inputs to the back panel so that I can switch several high level sources. After the initial gain stage I would have a buffered full range pre out and a buffered LP out to hook into my other amps as needed. I am thinking a pair of output tubes and maybe four 9 pin dual triodes (2 for VAS and 2 for preout/cross buffers) should do the trick.

The output stage is dead simple so it seems that it is not all that much more over the straight preamp however if it still seems like too much to bite off at once I could implement only the preamp part leaving the holes and mounting for output tubes and transformers empty and then place the resulting preamp/crossover in the tape loop of the main sand amp.

Oh, BTW given the inquisitive grandchildren and the yellow lab I would put a safety cage over the exposed tubes.

By putting the PS on a separate chassis I can keep the size of the amps down and it would seem to be a good move noise wise. The Organ chassis is large enough to put several PSs on it for additional amplifiers and such.

I really do appreciate all of the advice and your patience in helping me walk through the planning process.

mike