tubed pre-amp with solid state amp, problem???

[Lex]

A customer of mine was told this by someone. Fact or fiction? I have never heard of such a thing...

I was told it is not a good idea to put together a solid state amp with a tube pre amp. They said you can get back DC voltage that can destroy the speakers. The only way to stop this is to put DCblockers in the amp and that will take bass out. Have you ever herd of this or is this fantasy?

Thanks,
Lex

[JonMarsh]

In most cases this would be fantasy.

The typical problem with a tube preamp and solid state power amp comes in when the solid state power amp has a low input impedance (10K to 22K is not uncommon in bipolar input stage designs), and the tube preamp uses an output cap which into this "low" impedance rolls off the bottom end. In the past, it was not uncommon to expect the input impedance for a power amp to be 100K or higher; many vacuum tube designs are in the 220K or higher range. This makes a big difference in the roll off point on a coupling cap.

If a solid state power amp has a direct coupled input stage with no blocking capacitor, and if the tube preamp has output coupling caps which are "leaky" (i.e., lower quality electrolytics- not likely in a good tube preamp, which usually use high grade film caps), then there could be a situation where you have some coupled input offset (say, 20 - 50 mV), which is amplifiered by the forward gain of the power amp (30 typical), with a resulting DC offset of 1/2 to 1.5V, which may be enough to cause triggering of DC protection circuits, as well as cause a cone offset in the woofer at rest. That's equivalent to about a 1/2 watt DC drive. "DC Blockers" would be inline coupling caps to avoid the DC.

Not impossible to encounter this scenario, but fairly unlikely, and easy to trouble shoot.

Best regards,

Jon




Earth First!
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We'll screw up the other planets later....

[merc]

So, Jon,
are you learned or related to the Marsh who designed my power systems for my house???




Take Care,
merc

[Crimson]

Lex,
Tell your customer to do what I do: turn the preamp/source components on first, let it warm up for a few minutes, then follow that with the power amps. If I don't power up my system this way, and power the amps before the preamp, I get a good few seconds of loud distortion (potentially damaging to my speakers) at full output.




Q.

[JonMarsh]

What Crimson describes is not uncommon if a preamp doesn't have muting circuits and if the power amp doesn't either. Many circuits aren't in a stable output condition when first powered up; there are often current sources with bypass caps that have to be charged, and a large offset voltage or brief oscillation may be present at initial turn on, especially if a source is already connected and turned up.

OTOH, the all Denon preamp/monoblock system I handed down to my girlfriend can be powered up in basically any old sequency you want- it has a time delay of about 10 seconds before the preamp outputs are un-muted, and the same for the power amps when they're turned on- more like 15 seconds in that case. In fact, the Denon preamp is so "fussy" that it mutes briefly (a couple of seconds) when switching between different inputs. The Sony analog preamp I used to use is similarly equipped, as are Marchand Electronic crossovers, and my own custom DAC's. You can turn them on and off with active electronics downstream without problems; this is the only responsible way to design gear, IMO; the pennies saved by not implementing muting relays do the customer a real disservice. Muting relays can be shunt devices instead of series, so there's no reason for them to affect the signal path.


Hi Merc,

If you're referring to Richard Marsh of Marsh Sound Designs, no relation that I'm aware of, though interestingly he's spent 23 years at Lawrence Livermore Lab, which is in the town I've resided in for the last 17 years... small world and all that. "My" Marsh's come from the New England area, been around since the 1700's I hear.


Best regards,




Earth First!
_______________________________
We'll screw up the other planets later....

[Wireless]

One of the best sounding combinations I ever owned was a VTL PR9 tube preamp going into Classe solid state amps. The tube sound with the power of solid state. It makes a wonderful sounding combination.




John
My HT Picts

[Phil Rose]

Like the others said, it works but, you do have to make sure that compnents are compatible, which you should do anyway.

I'm running a Sonic Frontiers Line 3 into Bryston 7B-STs with absolutly great results. The Line has a pretty low output impedance of 90 ohms balanced and the Brystons are 20K ohms balanced. The amps are triggered on by the preamp and there aren't any thumps or turn-on problems.

[Kevin P]

I would expect that modern tube preamps are designed to be compatible with modern amps, both SS and tubed. Compatibility problems are more likely if you're using vintage gear, which was never designed with modern requirements in mind. The guy designing a pre-amp in 1955 would never have thought of making it compatible with 21st century SS amps.

KJP




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