[benw]

Hi, this might be a well worn newbie question, but here goes!

What is the function of a preamp, besides input selection and volume/tone/etc controls?

Reason I ask is that I’ve seen a lot of tube power amps around lately, that feature a volume control so as to enable the amp to be used as a quasi-integrated amp if you only have one source, presumably a cd player. Is this really a good idea?  Putting the convenience factor aside for a moment, is it feasible to run any line-level source into this one input with good results? If so, then why do complicated and costly preamps exist? ( and simple and costly for that matter) – if all that’s required is channel switching!  I know there must be  more to it, but I can’t think what it is.

In fact I’ve recently acquired a valve power amp with volume pots. Currently I'm using it with a solidstate pre , so I have two sets of volume pots. I’m wondering whether I should leave the volume pots in the power amp and build an external switchbox, or remove the volume pots from the amp, and build an external switcher/volume box, ie . “passive” preamp!  Or continue with an active preamp. Any thoughts on this?  Just the concepts , putting aside the quality of the actual components being used.

Well, more than one question there, but that’s the way it came out of my head!

Thank you.
BenW

[JohnR]

Hi Ben

Putting the convenience factor aside for a moment, is it feasible to run any line-level source into this one input with good results?

It depends on the output level of the source and the sensitivity of the amp. With most sources and amps, it will work fine.

I’m wondering whether I should leave the volume pots in the power amp and build an external switchbox, or remove the volume pots from the amp, and build an external switcher/volume box, ie . “passive” preamp!

It depends on the quality of the pots in the amp... my inclination would be to either modify the amp to add another pair of inputs and a switch, or bypass the internal pots and use an external preamp of whichever kind (active or passive) you prefer. But, the most sensible option if you're happy with the sound with a source plugged straight into the amp, would be to just build an external switchbox.

BTW what amp is it?

[benw]

Hi Ben

It depends on the quality of the pots in the amp... my inclination would be to either modify the amp to add another pair of inputs and a switch, or bypass the internal pots and use an external preamp of whichever kind (active or passive) you prefer. But, the most sensible option if you're happy with the sound with a source plugged straight into the amp, would be to just build an external switchbox.

BTW what amp is it?

Hi John, it's this monster!

http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&item=3089847498

it's a little rough cosmetically, but I kind of like that. The guy I bought it from has been very helpful with advice, but I thought I'd try a wider circle, so to speak. I am new to this game, having just used decent mid-fi stuff for years, your Rotels/Nads etc, but I saw this on ebay and it sparked my interest. I have an old Electrocompaniet preamp, maybe the first model they made, but I want to try a simpler approach more in keeping with the valve amp. BTW the pots in the triode are Bournes plastic film type.

One thing I have to do before I use it too much is install some capacitors at the input to block any residual voltage. Don't know much about that issue either, just beginning to investigate it. From my initial research Auricaps would be a good choice. I also have to build boxes for the output transformers, they are currently sitting on the floor, in front of my speakers (Kelly KT3s) protected by plastic plant pots!

cheers,
Ben

[Dan Driscoll]

Hi, this might be a well worn newbie question, but here goes!

What is the function of a preamp, besides input selection and volume/tone/etc controls?

All that, plus gain, if necessary. If a 'preamp' does not have a gain stage it is typically called a linestage.

Reason I ask is that I’ve seen a lot of tube power amps around lately, that feature a volume control so as to enable the amp to be used as a quasi-integrated amp if you only have one source, presumably a cd player. Is this really a good idea?  Putting the convenience factor aside for a moment, is it feasible to run any line-level source into this one inp ...

If the output level and impedance of the source component match well with the amplifier, and the amp's volume control is good, yes, this can be a very nice configuration. Whenever you can eliminate connections and circuitry it is generally considered a 'good thing'.  

The problem of course is that very few people actually only have 1 source component. Most have at least 2, many have 3, and some have 4 or more. Many people also want or need a tape loop. Tape loops are a common feature on preamps and integrateds, but never found on power amps.

[benw]

If the output level and impedance of the source component match well with the amplifier, and the amp's volume control is good, yes, this can be a very nice configuration. Whenever you can eliminate connections and circuitry it is generally considered a 'good thing'.  

The problem of course is that very few people actually only have 1 source component. Most have at least 2, many have 3, and some have 4 or more. Many people also want or need a tape loop. Tape loops are a common feature on preamps and integrateds, but never found on power amps.

thanks Dan. So if I'm happy with the amount of gain I get just from my poweramp (with built-in attenuater), all I need is a device for source switching, tape loop if required, and to "adjust" the impedence. What I need to know is how necessary is this impedence matching function. Don't manufacturers adhere to some kind of standards so their products' impedences are in the same ballpark? When is an impedence mismatch likely to show itself with professionally designed products and what ill effects can there be? Preamps seem awfully expensive for the little they are required to do - from my perhaps naive perspective!

is there a general rule about impedence. eg. allowable ratio Source/input  or one must be greater than the other by a certain amount or some such rule.


btw I have done a bit of browsing and found some definitions such as these (below) at audio-direct.com but am struggling to apply them to my requirement for a device that sits between my source and power amp, and gives me channel switching capability. Hopefully something simple I can construct myself.

cheers,
Ben

audio-direct.com  "Impedance. The electrical characteristic of an electronic device that reflects the voltage developed in response to a current supplied to the device. Speakers typically display a low 4 ohm or 8 ohm input impedance to accept high power drive from reasonably low signal voltages. Microphones typically present a high output impedance to develop a sufficiently high signal voltage from their low internal power generation.(See also Impedance Matching.)

Impedance Matching. Matching a driven load impedance to that of the source for maximum power transfer, as often implemented through transformer coupling for impedance transformation. A feature incorporated in many in-wall volume controls through transformer tap selection. Becomes especially important where multiple speakers share a given audio signal line, as each speaker added alters the net load impedance. "

[Dan Driscoll]

thanks Dan. So if I'm happy with the amount of gain I get just from my poweramp (with built-in attenuater), all I need is a device for source switching, tape loop if required, and to "adjust" the impedence. What I need to know is how necessary is this impedence matching function.

In some cases impedance matching can be very important, particularly with tube gear or when mixing tube and solid state equipment.

But if the source and the amp are both solid state, then you can most likely just hook the source up to the amp and be ready to go.

Don't manufacturers adhere to some kind of standards so their products' impedences are in the same ballpark? When is an impedence mismatch likely to show itself with professionally designed products and what ill effects can there b ...

There are general practices, but TTBOMK, there are no fixed industry standards for input and output impedance characteristics. Keep in mind that the design of a particular device often dictates a general range of impedance characteristics. Generally, you want the input impedance of an amplifier to be at least 10x greater than the output impedance of the preamp (or CD player) that is driving it.

If the input impedance of the amp is too low it will load down the preamp. If the input impedance of the amp is too high it will look like an open circuit to the preamp. Both of these conditions will cause ditortion, weak singal strength and generally poor sound quality. But there is a very wide range that is acceptable, particularly for solid state gear. As I said before, tube equipment is much more sensitive to impedance matching.