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Bi-amping with tubes?

AdamAnt316

AdamAnt316

Collector of heavy things
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Hello everyone. I recently picked up some Dynaco tube amps, a pair of Mark IIIs, and a ST-35. Since my Paradigm Studio Monitor speakers have separate binding posts for the lows, mids and highs (all going through the internal crossover), I figured I'd try bi-amping. I connected each Mark III to the low driver terminals, and each channel of the ST-35 to the mids and highs (jumpered together). At low volumes, this seems to work fine. However, at high volume levels, the bass seems to overwhelm the mids and highs, and things get a bit muddy. Should I try using the ST-35 for the highs alone? Or am I going about this all wrong?

Also, what should I do regarding the impedance taps on the amps? The Paradigms are 8 ohm speakers, and the low/mid/high terminals are usually jumpered in parallel. Would the 8 ohm taps of the amps be sufficient, or should I use the other taps instead? The manual says nothing about impedance differences when bi/tri-amping, but they were designed when solid-state amplification was in vogue, so I'm not surprised. Anyway, thanks in advance!
-Adam
 
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If I remember right, none of these amps have volume controls do they? You really need at least one set of amps that do, whether LF or the HF, and having both HF and LF with volume controls is your best bet, but you need a way to match the output levels sound wise..
 
It sounds like you need to lower the input level to the MK IIIs since the gain is probably a bit higher. Knock together a haler passive pre and that will give you some level control. I would almost bet each amp would put out its maximum power at ~the same input level, which would make the bass alot louder.

As far as the impedance, that is resistance with respect to freqency, and is only a nominal level anyway (the graph could vary anywhere from 4 to 200 ohms for an 8 ohm rated speaker, easilly). Since each driver is handling seperate freqency ranges, the impedence across each will still be a nominal 8 ohms since they really arn't runnint in paralel (well, sort of but not really).IOW, keep using the 8 ohm taps. :D

Curses, there are no pictures of bass alots.... I shall have to make one later.
 
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similost said:
If I remember right, none of these amps have volume controls do they? You really need at least one set of amps that do, whether LF or the HF, and having both HF and LF with volume controls is your best bet, but you need a way to match the output levels sound wise..
Click to expand...

Indeed, neither of these amps has gain controls. I figured that things would be OK, since neither of the Mark IIIs was near to it's full RMS output power. However, I guess I'll have to track down some sort of crossover or equalizer before I can make another attempt at this. Also, the ST-35 could probably use a new set of output tubes.
-Adam
 
Regardless if you balance the gains, the ST-35 won't be able to keep up even at less than full output of the MK IIIs.

The voltage gain difference between these two amps seems to be a bit more than 1dB. Some difference, yes, but not a huge one and likely not the issue if it works fine at lower volume but falls apart at higher volumes...that's almost certainly due to a small amp not keeping up with bigger amps.
 
You're feeding the full spectrum to both amps, so the smaller one is probably clipping early due to low-frequency content that's not contributing anything to acoustic output. Simple fix: Insert a passive high-pass filter inline to the mid/tweet amp.
 
Or even better, an active crossover. While you can get away with just a high pass filter in between the preamp and Stereo 35. Getting the lows out of that amp will go a long way toward cleaning up the sound by pulling that amp down out of clipping. Ideally you would also have a low pass filter in front of the MKIII's as well, but that is less critical then the other.

Good Luck.

Shelly_D
 
What he said. Do your crossing ahead of the amps. Magnavox did a bi-amp rig in the late 1950s. The bigger one had 6 6v6 tubes in push-pull-parallel for lows, but only two for highs. The smaller one had a quad of 6v6 tubes for low, and a single for high. To put that in terms of power output, the larger amp put out roughly 40 watts for the low frequency side, and about 15 for highs.
 
gadget73 said:
What he said. Do your crossing ahead of the amps. Magnavox did a bi-amp rig in the late 1950s. The bigger one had 6 6v6 tubes in push-pull-parallel for lows, but only two for highs. The smaller one had a quad of 6v6 tubes for low, and a single for high. To put that in terms of power output, the larger amp put out roughly 40 watts for the low frequency side, and about 15 for highs.
Click to expand...

X 2, as they are mono.:D
 
yeah, should have mentioned that. 40 per channel for the lows, and ~15 per channel for highs.
 
A friend of mine has suggested putting .047µF or somesuch capacitors in series with the inputs of the ST-35 to cut down on the low end stuff it's trying to amplify. What do you think? :scratch2:
-Adam
 
Find yourself the schematic for a Magnavox AMP-150 and have a gander at the crossover setup used there. Its an LC network with adjustable chokes to tweak the cross points. You may be able to duplicate that or something like it to get what you're after. Being able to really tweak the crossover points would help quite a bit. No point in feeding sound frequencies to the amplifier that the speaker is not going to reproduce anyway.
 
Alot has do with what speakers you are using horn speaker for the top end , 15 to 20 watts is more than enough , and 15 inch woofer on the botom end the MKII's would be a good match . Knowing what speakers you are using would help alot to help figure out what to sugest to you .
 
battradio said:
Alot has do with what speakers you are using horn speaker for the top end , 15 to 20 watts is more than enough , and 15 inch woofer on the botom end the MKII's would be a good match . Knowing what speakers you are using would help alot to help figure out what to sugest to you .
Click to expand...

The Paradigm Studio Monitors use two 8" woofers, a 4" midrange, and a titanium(?) dome tweeter. Not sure on the exact specs off-hand, but believe they have an efficiency rating of 90dB/1W/1m. Here's a review of the speakers:

http://isaacvw1976.hubpages.com/hub/Paradigm-Studio-Monitor-Review

And here's a picture of one like mine:

paradigm_sm1.jpg
 
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Putting a crossover ahead of the amp does cut the LF component of the voltage swing, but it does not fix the issue because the (main) issue is the crossover in the speaker.

The selection of drivers and design of the crossover in the speaker have established the ~same voltage sensitivity for all sections of the speaker - this is the problem, the voltage sensitivities.

A 60W amp can swing roughly 22V while a 17W amp can swing roughly 12V. So, since the voltage sensitivity of all crossover input sections of the speaker are the same, your goose is cooked when you reach the 12V limit of the small amp, regardless of anything else.

The only way around that is to strip/modify the in-speaker crossover or drivers to make the voltage sensitivity of the mid and highs greater (by roughly 6dB) than the lows. With (roughly) 6dB greater sensitivity in the mid/high of the speaker, the 17W amp could keep up with the 60W amp.
 
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All the talk about voltage sensitivity and being hosed by the crossovers in the speakers presupposes that you continue to use the crossovers in the speakers as originally designed. When biamping with 3 way speakers and 2 way amps there are a few things you must do and a few things that you can keep in mind.

What to do:

Trance out the crossover in the speakers to see if there are any resistors in it to pad down the midrange and tweeter to meed the level of the woofer. One or both might very well be padded down. Possibly by differing amounts. What you need to do is separate the woofer from the crossover and bring it out to its own terminals on the back to allow it to be wired to the amp directly. Then look at the remaining crossover network and look for pads. If for example, there is a 3 dB and 5 dB pad for the mid and high level drivers, you can remove the 3 dB pad completely and reduce the 5 dB pad to 2 dB. The math to do this is not terribly difficult and the components are just a few power resistors.

What to keep in mind:

While it is perfectly true that the smaller amp might clip at a lower voltage, say 12 volts in the example sited, this is also true: There is more power contained in source material in the bass then in the midrange and treble ranges. Since electrical power can be measured as V squared / R, lower power requirements translate to lower voltage requirements for the same listening level. So to put it in practical terms, when the higher frequency amp reaches the 12 volt output level, the low frequency amp could well be putting out 16 or 18 or more volts simply because that is what the music program is demanding.

There are many biamp installations running successfully with lower power amps used for the mids and highs and higher power amps for the lows, simply because of the difference in the power demands made by the differing frequency spectrums. Now I know that there are some program sources that this does not necessarily hold true, and on those you will see the limitation clearly, but much more music it does hold true and you will see the benefit.

Shelly_D
 
I have run several amps in my bi-amp system. I usally go 3x power for the lower end. Currently running Heathkit W6s and W5s split is at 700hz.
 
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