Tube pre-amp with ss amp?

[Feanor]

This is the first of three factors that determines compatibility. Low output impedance of the source is also desirable to provide a suitable input to output multiple. See attached link for calculator to determine potential HF rolloff.

DACT calculator

Balanced or not, the second factor is cable capacitance. The lower the better.

The final factor is gain. Some folks say they lose dynamic "punch" with passives. I think they simply don't have enough gain. My CDP has a 4 volt output driving 1.3 volt sensitivity amps. I can clip them at will.

rw

Perhaps I'll try some of the math sometime. I do know that the Monarchys' input impedance is supposed to be a good match for passive preamps but I can't quote the spec offhand.

I find the "punch" of the Monarchys at least as good as their predeceasor, a Bel Canto eVo. And I never use a volume knob setting higher than 3 o'clock, necessarily less than unity gain. As for HF roll-off, maybe I just don't hear it as I'm deaf much above 10kHz.

[E-Stat]

Perhaps I'll try some of the math sometime.

All the calculator needs to display response curves is:

1. Amp input impedance (kohm)
2. Cable capacitance (pf)
3. Attenuator resistance (kohm)

edit: Unless I'm playing a particularly low level recording like a Telarc, I usually get full output at around -12 db.

rw

[Feanor]

All the calculator needs to display response curves is:

1. Amp input impedance (kohm)
2. Cable capacitance (pf)
3. Attenuator resistance (kohm)

edit: Unless I'm playing a particularly low level recording like a Telarc, I usually get full output at around -12 db.

rw

So ...

• Monarchy SM-70 Pro amp input impedance is 100kOhms;
• Blue Jeans balanced cables are 13pF/foot, hence 78pF for 6 foot
• Attenuator resistance is -- what? my Adcom GFP-750's output impedance, presumably at 0 dB attenuation, is 1.2kOhms, which is much lower than the calculatior allows.

It would seem there is essentially zero attenuation any where in the audible range. Right??

[E-Stat]

It would seem there is essentially zero attenuation any where in the audible range. Right??

I would say so. What is the output impedance / gain of your source(s)?

rw

[Feanor]

I would say so. What is the output impedance / gain of your source(s)?

rw

My Sony SCD-CE775 says 2 Vrms and >10kOhms;
My M-Audio Audiophile USB say +2dbV, no o/p impedance specified.

I'm curious as the "Vrms" as opposed to "+dbV". (I don't understand these things )

[E-Stat]

My Sony SCD-CE775 says 2 Vrms and >10kOhms

The output is quite common, but the impedance is not. Perhaps what you are looking at is the intended input impedance of what is driven. The output of my Pioneer PD-54 is 600 ohms (another typical answer) while the GamuT CD-1 is 75 ohms.

My M-Audio Audiophile USB say +2dbV, no o/p impedance specified. I'm curious as the "Vrms" as opposed to "+dbV".

That works out to something like 1.25 volts. Here is a conversion tool

rw

[Feanor]

The output is quite common, but the impedance is not. Perhaps what you are looking at is the intended input impedance of what is driven. The output of my Pioneer PD-54 is 600 ohms (another typical answer) while the GamuT CD-1 is 75 ohms.

That works out to something like 1.25 volts. Here is a conversion tool

rw

Interesting. Perhaps your right about my CDP but the specs weren't any more specific.

Anyway, I have no beefs using my Adcom in passive mode.

[StanleyMuso]

Ealier I asked a questiion about the need for constant fiddling that I was warned about by somebody to keep an amp at its optimum. I have been doing some research, and apparantly that "fiddling" my informant was refering to is called biasing. I was told I would need to get a voltmeter and some insulated screw drivers to carry out this "biasing". He says he does it about every three months, but that people who are really anal do it on a weekly basis. I was not able to get more info from him - another customer came in and he was being impatient with me in the first place, as if I was a moron asking obvious questions. Don't want to go back to him afer that, and I certainl won't be buying anything from him!!!

So, could I impose on your generosity once more? Could someone explain in layman terms what on earth is biasing, and is it a very onorous? I'm used to ss equipment which doesn't need constant tuning to keep it at its peak. Please bear in mind I am not a technical person, so would I be able to carry out biasing myself, or would I have the constant expense of getting somebody else do it for me?

Once again, thank you all for a very interesting discussion.

[Dusty Chalk]

Basically: tubes are complicated. Each one is individual, and you're setting one of the middle voltages (that setting will be different for every tube). So you need to know what your "bias voltage" is, set the voltmeter on the bias points, and then change the bias (usually entails turning an inset screw -- you'll need to know where that is, too) until the voltmeter reads what it should.

There's usually several per amp -- one per tube.

And it needs to be done while the amp is on, hence the need for insulated screwdrivers. And with no music playing.

Oh, and: he's high -- you don't need to set your bias every three months. You need to set it once when you get it. I'd check it again in a month, and if it needs changing, then check it again a month after that. When you check it and it doesn't need changing, then it's settled down, and then I'd only check it every 6 months to a year after that. Then you only need to check/change it again if (a) you move the amp (new voltage), or (b) you install new tubes.

And there are some amps that are auto-biasing. You never need to do those. I used to have one of those. I miss it.

[Mr Peabody]

First of all you are talking about buying a preamp, to my knowledge there is no preamp that needs biasing, that is a power amp adjustment only. And only the output tubes. Biasing is irrelevant to what you want to do.

If you ever think of buying a tube power amp, biasing is not a big deal if you buy the right equipment. Either entry level integrated or very expensive tube amps can be found with auto-bias where you have no worry. Conrad Johnson has a built in LED for each bias adjustment. No meter needed. You just turn the screw until the light comes on then slowly back it off until the light goes out and you are done. Actually, from my manual and every dealer I've spoken to biasing only has to be done when you initially set the power amp up and if you move it or do anything to effect the electric going into the amp and, of course, when you replace one of the tubes. My power amps are now at the factory being converted and I will double check how frequently they recommend checking the bias.

I am not a tinker either so when I was looking for a tube amp easy biasing was one of the criteria I kept in mind. I did not like the idea of using a voltmeter. Off the top of my head I can't remember who but other manufacturers offer easy bias adjustments as well. Don't sweat it.

[StanleyMuso]

I really appreciate it.

Obviously I got myself confused, and started seeing complications where they don't exist. I guess the salesman didn't realise I wanted to know about pre-amps and not amps, and I didn't realise that he was talking about power amps. That's the trouble when you first start investigating something new - you don't know what you don't know, and don't know what questions to ask. I have since found some on-line info and I'm beginning to gain some understanding. I'm going to see if I can find a decent book or two on the subject and do some reading up.

Once again, thank you for the input. It encourages me to explore further.

[Mr Peabody]

I talked with one of the guys from C-J today and asked about how frequent to bias a power amp tube, he said probably once a year is fine or every 3 months or so if you are OCD. The impression I get from talking to different people the more frequent check may be necessary, or recommended, as the tubes get older.

It also seems that tubes don't necessarily wear at the same speed. Not long ago I posted some where on here that I planned to switch all the output tubes when one goes bad but I've since changed that plan and will just change them as they go out.

Stan I think the type of info you are looking for will be difficult to find in a book. If you do find one, let me know too. You are fine doing what you are doing. Don't let one jerk sales person prevent you from asking questions. Have you any brands in mind to investigate yet?

[StanleyMuso]

Have you any brands in mind to investigate yet?

No, not yet. I'm right at the beginning of my exploration stage, and don't really know the pros and cons of various models out there. I notice that Chinese manufactured units seem reasonably priced, but I have no idea about their quality.

In addition, my thinking is evolving on the matter. I will start with a pre mated to my Musical Fidelity, and perhaps if I like it, eventually go the whole hog and get a power amp as well. In that case I would run the tubed equipment for vinyl and CD stereo only, and dedicate all the solid state stuff to home theatre and surround. However, all this is in the future, as I am low on funds at the moment, and my knowledge base is still not where I would be comfortable to make such a plunge. I am cautious by nature.

[Mr Peabody]

I understand you being cautious, you want to be sure you are making the right move for you. Going with a tube preamp will be a safe start.