convince me the FP is not a glorified switch

[denti alligator]

Not that that's what I think ... i.e. that it's only a switch with gain ... but I'm wondering how you would characterize the addition of the FP to your system in terms of quality (not quantity, i.e. loudness). Obviously it's function as a switch is a nice advantage, too.

I'm even more curious about how the Extended FP upgrade contributes to this quality. Does it improve on what's good about the FP or does it make up for deficiencies that the FP introduces? It would be a major investment for me, so I want to be sure I understand what I'm paying for.

[corndog71]

It sounds like you've never used a preamp before.  Is this true?

[Paul Joppa]

A preamp provides four functions:

1) selector switch,
2) volume control,
3) impedance converter
4) gain

In most systems most of the time you don't actually need the gain.

Most sources, especially tubed sources, work a little better into a high impedance. You need a low output impedance to drive long cables (over 2 meters).

A "passive preamp" provides a medium impedance at input and output, and performs the first two functions. Here's an example: http://www.goldpt.com/sa4.html

Does that help?

[denti alligator]

It sounds like you've never used a preamp before.  Is this true?

Yes.

Can you explain impedance / impedance conversion and how/why this is critical to the sound I hear and also how it relates to the other parts in the chain (source, amp, speakers)?

Yeah, I'm totally ignorant. But I want to learn.

If I understand you right, this is the main function the pre-amp offers.

[Paul Joppa]

Actually, the switching and volume adjustment, along with the set of RCA jacks for connecting all the parts of the system, are the main function. At least, they represent a large part of the cost! That's why I included the example of a passive preamp. But here I'll try to describe the impedance issue.

Source components include computer outputs, CD players, phono preamps (for use with turntables), iPods, tape decks, radio tuners, etc. It's impossible to be absolute about this, but most solid-state devices will drive a load of 10K ohms or greater with good performance. Many tubed devices however require a higher impedance load, usually 50K or 100K minimum. For example, Seduction is designed for a 50K load, though it works acceptably into a load as low as 15K.

A passive preamp, i.e. an isolated volume control, is usually a potentiometer of 5K to 25K impedance. (10K is popular.) This means it may or may not work satisfactorily with a tubed source component.

At the other end, the equivalent impedance of the output of the preamp must be low enough that it does not interact with the capacitance of the cables between the preamp and the power amp. This is a combination of treble loss and excess current demand, so there are two calculations involved. A simple potentiometer level control has a maximum output impedance of 1/4 the input impedance, i.e. 1250 to 6250 ohms. In practice, this means a cable of more than 2 meters length will audibly degrade the sound. Most preamps will have an output impedance of 600 ohms or less.

[Paul Joppa]

I have already described passive preamps, which provide the switching and volume adjustment functions without gain or impedance conversion. They are often practical and some audiophiles find them preferable to active preamps.

If your amplifier already has a volume control and selector, then you don't need another one. This is usually called an "integrated amplifier". For example, Stereomour provides a selector and a volume control inside the amplifier and no preamp is needed.

If you have only one source, then you don't need a selector. For example, SEX, Crack, and Smack have internal volume controls.

If you just need a switch, there are a few on the market; I have two models that I picked up at Radio Shack some years ago. They were cheap, maybe not the best or most reliable but they did the job. (Currently they are stashed in the basement because I have a Foreplay.)

Some sources have internal volume controls of their own, so the amplifier does not need a volume control. An iPod is an example. Just aware that some of these use digital level adjustments which often damage the sound quality.

[denti alligator]

Actually, the switching and volume adjustment, along with the set of RCA jacks for connecting all the parts of the system, are the main function. At least, they represent a large part of the cost!

OK, this all makes me feel stupid. So please excuse the further questions.

First off, if the main functions are switching and volume adjustment, then it IS essentially a switch with volume adjustment, no? Except that I can't distinguish the volume adjustment from gain. I set the knobs where I want them and leave them there, no? It's the main amp I use for adjusting the volume.

But back to impedance. Take it down two or three levels. I still don't understand exactly what the levels mean. Let's get practical. I have three sources: CD player; PC; and phone. The phono will hopefully soon be sent through a Seduction. Now the volume of all three sources will be about the same. I plug these (through a cheap switch) into my SEX amp and they sound great (well, the PC could use a DAC, but ...). Where does (would) impedance come in?

[Paul Joppa]

Yes, you have it exactly. The main function is a selector switch and signal level adjustment.

On impedances: the input impedance of the SEX amp is 100K (100,000 ohms). That's the load that any of your sources see. Sources will be happier the higher that impedance is, and each source will have a minimum load impedance, below which is does not perform well.

Seduction wants to see at least 50K, so it's happy.

CD probably wants to see at least 10K, so it's very happy.

Phone wants to see at least 10K, so it's very happy. If you are using the headphone output instead of line out, it would be happy with anything over 32 ohms (0.032K).

Everybody is happy, so your system should rock. All you need is a cheap switch. If you can't find one, we'll help you make one - this forum is very helpful!

================

Now gain and volume is another confusion. I have a whole white paper on the subject here:

http://www.bottlehead.com/loosep/signals.htm

All the knob and potentiometer does is reduce the signal voltage. Notice that it does NOT set the output voltage! It only sets the ratio between output voltage and input voltage. It cannot be calibrated in volts, only in the ratio of voltages.

In a normal preamp, that reduced voltage is then amplified, usually by 3 to 5 times. The control is not linear, but somewhere in the top half of its range the potentiometer reduction is 1/3 to 1/5 of the incoming signal, so the output voltage is often the same as the input voltage. Without the amplifier after the potentiometer, the output is never more than the input.

If everyone followed a set of standard voltages at various points in a system, we could design around this and everything would work together. Unfortunately that's not the case. An iPhone has (I think) a peak output of 0.95 volt. A standard CD player has 2.0 volts, but some are more, up to 5 volts. I remember back when cassette tapes were still in use that some decks would put out 0.1 volt. To make matters worse, some components are rated for peak output, while others are rated for a kind of average, which in practice is about 1/5 of the peak voltage.

In most modern systems, every component has more gain than is needed, and the preamp (or integrated amp, like SEX) is used to reduce that gain to what works.

===============

Finally, I should say that some people do prefer the sound of a system with a preamp in it, even when there is no technical need for one. But this is a matter of subtle perceptions. There are various explanations that may or may not be relevant. In practice, you would likely hear a small difference, and might prefer one to the other. There is much discussion on the various forums about this. But I say again, it is subtle.

[corndog71]

I did an experiment once for a friend.

Source is Rega Planet CD player.

Amp is the amp section of my Bryston B60 integrated.

I switched between:
1. a passive preamp made from Bottlehead's Sweetest Whisper controls.
2. The preamp section of my NAD 304 integrated amp.
3. The preamp section of the B60.

Through the passive the sound was very clean and detailed but lacked oompf or dynamics. The music was playing but nobody was dancing.

Through the NAD the sound was more veiled yet it did have a bit more drive than the passive.

Through the B60 there was much better resolution and dynamics over the NAD.  The passive had very good resolution and was very close to the B60 but lost so much in the way of dynamics that the B60 was the clear winner.  It shouldn't come as a surprise as it was much better built than the others.

My point is that while the main functions of a preamp are switching and volume, the underlying function is to better drive the signal from the source to the amp.  That extra gain stage can help retain the drive and dynamics of the music.

It's often been written that the sex amp is great by itself but sounds even better with the Foreplay III in front of it.  I've found this to be the case with the quickie as well. 

[Grainger49]

Sam,

I have used passive preamps, a pot, for years.  It was simple and didn't provide a good load to the amp.

When inserting an active preamp you get drive.  The output section is capable of delivering current that the passive just doesn't have.  My music became more "alive," more dynamic.  Of course it has to be well designed because any active stage introduces noise and distortion.  That applies to SS and tubed stages.

[Laudanum]

I agree with Corndog and Grainger.  I really needed the source selection and wanted the second output for future versatility and I had some ideas already regarding building for 6SN7's and some synergy related to those tubes but I was still on the fence.  It was Graingers take on the "drive" that was the final deciding factor for me to go for the FPIII.   It was definitely the right decision, for me.

[denti alligator]

It seems to me that with the Paramounts the Foreplay would be ideal. Three inputs into the FP III, outputting to the Paramounts for speakers, and using the second output for a Crack/Smack/SEX for headphones.

That would be a dream system, especially if one of those inputs was an Eros, and the other a BH DAC

But for my current modest set-up, I'm going to keep my simple switch, save up for the Seduction, then for the iron upgrade to the SEX, and by then the DAC will probably be available...

[Grainger49]

The Fortes should be happy with the power of the SEX.  If you are happy with the Forte speakers then don't worry about the amp.  I agree the iron is a good place to get added range and finesse. 

[John Roman]

Excellent thread gentlemen!