Adcom Compatibility Question

[MacAddict]

I'm considering building a Foreplay III to replace my dying preamp. My Amp is an aging Adcom 555II, and the speakers are Kef model 104/2. I've had this system for some time, and it sounds great, but I'd like to replace the preamp (at least to start, with possible Bottlehead power amp in my future, should things work out well).
Can anyone help me determine compatibility with the proposed amp and speakers? It would be a shame to build a great preamp and not be able to use it with my current system. Thanks a ton for any advice!

[MacAddict]

Lots of readers, but no replies. Here's more info on the speakers and amp - will this help? Thanks!!!

Kef 104/2 Speakers:

Crossover Point    150Hz, 3kHz
Nominal Impedance    4 ohms
Characteristic
Sensitivity Level    92dB
Amplifier Requirements   25-200W into 4 ohms
Maximum Output    112dB

Adcom 555 II Amp:

200 watts continuous average power into 8 ohms
325 watts continuous average power into 4 ohms
600 watts continuous average power into 8 ohms (bridged)
- Freq. response: 10 - 20,000Hz
- Gain: 27dB
- S/N Ratio: 200 watts into 8 ohms, >110dB
- Input Sensitivity: 200 watts into 8 ohms 1.75V rms

[JC]

would be helpful: Input Impedance of the power amp.

You may have to play with some resistor values at the input to the FP to get the gain structure where you want it, but I believe that is already taken into account in the kit.

Other than that, it is recommended that the power amp have an input impedance of 10 kOhms or greater, although you could probably get away with less.

[Grainger49]

I had thought that PJ would wade in here.  He can calculate the output of your speakers with all the information you have now given.

The FP III has a lot of gain.  When building it there is a point at which you can "pad" the gain.  That means lower it for all or just some inputs.

Many, but not all, transistor amplifiers have a high sensitivity.  That means they will reach full power output with a lower input voltage than most tubed amps.  The specs that you have supplied don't indicate an overly sensitive amplifier, if I am remembering it correctly.  

If you purchase a FP III kit the kit is supplied with a set of resistors that allow you to tune each input so that you are running in the most advantageous range of the volume control.  That range is above 12:00 for loud and satisfying listening.  That also gives you more clicks on the volume control for quiet night listening.

For instance my Seduction has lower output than my DAC.  I have the DAC input padded so that I don't have to run the volume up and down a great distance when changing inputs.  It also keeps me from getting blown out of the room when I change back to CD and don't remember to turn down the volume controls.

I am not aware of the FP III being incompatible with any system with a few standard tweaks such as an attenuator in line to the power amp.  So I think that the FP III would sound pretty good in your system.  Those are well respected components.

[Paul Joppa]

Rated 92dB, so 102dB peaks will take 10 watts. That amp makes 200 watts with 1.75v in, so it will make 20 watts with 0.55 volts input. A typical CD player will have 2 volts peak output, so you need a preamp with approximately 12dB loss; the Foreplay (like most preamps) has a gain of about 10dB. That calls for a lot of attenuation! I would suggest at least 20dB of attenuation at the amplifier input as the best approach. The combination of a high powered amp with a high sensitivity speaker always has this problem - if you search the archives you'll find many versions of this discussion. Also see my white paper on signals and noise, on the Bottlehead site under "Community".

[MacAddict]

Thanks a ton guys; this information is very helpful. I've been looking for affordable attenuators - the largest value I've found is 12db, without a major investment. Certainly food for thought. I'll search the forums for more info about attenuation before I proceed, but clearly there would be no shortage of power with the proposed system!

[Grainger49]

Since you are going to be soldering on the FP III you can put the attenuator inside the amplifier at the input RCA jacks.  It only requires two resistors at each output RCA Jack.  This makes it the same as an inline attenuator.  Since I don't have my scientific calculator near I'll have to give you a suggestion later or PJ will chime back in.  IIRC, PJ isn't fond of the sound of those commercially available attenuators.

[Paul Joppa]

The commercial attenuators are fine; the main problem is that they are usually low impedance. If you can find some at 10K ohms or greater, they will do the job. I use the 12dB ones myself; mine are "Harrison Gold" brand. There is a pricier and quite possibly better  product out there; I can't remember the name but I think they are from England. The cheaper ones are usually made for car stereos and can have impedances as low as 600 ohms.

Some day I'll put a "Sweetest Whispers" kit in a small box, set up as a 15K ohm passive preamp with short captive output cables, just to illustrate another way to approach this issue.

[Dyna Saur]

PJ inscribed:
 

Some day I'll put a "Sweetest Whispers" kit in a small box, set up as a 15K ohm passive preamp with short captive output cables, just to illustrate another way to approach this issue.

I did very similar that with the SWs leftover from my 6SN7 & step-attenuator modded FPIII.

Put the two SWs onto a small  cast aluminum box, along with two pairs of RCAs, and I have been using it in conjunction with various tube amps which do not have their own volume controls. 

This passive pre / attenuator box  works very well, and  it is an invaluable piece of test equipment, especially when I'm  firing up a newly built amp for the first time, so I don't slam my eardrums together in that great empty  hollow space in between ;-)  The eleven steps are more than sufficient, and thois allows one to (as Spinal Tap sez) "turn it up to eleven".

/ed B in NC

[Doc B.]

Yeah it is useful. We keep an old Foreplay II chassis plate with just the 15K sweetest whispers and input and output jacks in the lab.

[MacAddict]

The idea of using a pair of SWs to make a passive preamp is very much what I've been thinking about, as I've not felt a great deal of confidence in the commercially available attenuators I've been able to find online. I'm glad to see it mentioned here as an option - seems easy enough to build even for a newbie such as myself. Thanks to all for your helpful comments.

[ssssly]

Used my FPII with SW installed with an ADCOM 555 and 89db speakers. The volume control was a bit jumpy but unless you want to listen very quietly it was OK. I had decent steps in between moderate listening and loud. Was pretty much full volume a click or two past half way. If the gain on the FPIII is similar to the FPII I would say give it a shot to see if it will accommodate your listening levels and if it doesn't then modify it to decrease the gain to better match your system.