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Built my Foreplay tube preamp (1 Viewer)

Saurav

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This is probably not of interest to everyone, but some people had asked me to report my results. Well, I got it done a few days ago, and am now starting to mod it, so I figured this would be as good a time as any to report on my results.
Product information:
It's a tube linestage/preamp. Go to www.bottlehead.com for more information. www.audioasylum.com has a forum dedicated to Bottlehead products, and there are tons of threads on all aspects of building, troubleshooting and upgrading the Foreplay preamp. Cost: $150 for the full basic kit, more if you want upgraded parts, obviously.
Other equipment:
Rega P3/RB300 turntable with Dynavector 10x4 cartridge
Radio Shack "Little Rat" battery powered phono stage
Denon DCM-270 CD changer
NAD C-340 integrated amp (now used as a power amp)
Paradigm Mini Monitors
Sony SA-WM40
Radio Shack Gold Series interconnects all around
Generic 12AWG speaker cable
Construction:
Fairly simple and straightforward. Some of the mods are going to get a little tricky, but the basic preamp is really easy. It would have helped if I'd been a little more patient, because I goofed things up a few times. I'm really grateful my NAD amp has some kind of protection built in - I sent that into shutdown a few times before I got all my grounding done correctly.
Cons:
I still have a little bit of hum/buzz from my speakers. It's audible from my listening position only late at night, but can be heard about a foot from my speakers. Since grounding/shielding are a bit of an art and no two situations are the same, this will take some chasing down, and that's the next step for me.
Pros:
The sound, obviously. This kicks the NAD's preamp section completely out of the room. The biggest difference is in the bass - there's much more texture to it, if I can call it that - bass guitars don't just sound like bass notes any more, I can hear much more of the tone of the guitar. Treble and midrange are smoother, voices are more realistic. Soundstage/imaging seem about the same, but that's not something I pay much attention to anyway. I played a movie through it last night (analog outputs from my DVD player) and that sounded much clearer, in fact, the improvement seems to be more for the DVD player than for any of my other sources.
It also plays with a greater sense of control, that's the only way I can describe the effect. My wife noticed it too - she said it sounded more powerful even at the same volume. No, I didn't check with an SPL meter so maybe I'm just playing it louder, but now I've been listening to this preamp across its volume range, and the effect is consistent - it just sounds more authoritative than the NAD. I'm not sure what exactly is causing this effect, but it's there.
And that's about it. Feel free to email me with questions, or post them here.
 

Brett DiMichele

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Sep 30, 2001
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Brett
Saurav,
Just what I wanted to hear...
I think I have my mind made up that I will be coupling a
Forplay with 4 AudioSource AMP5.3's or 4 nOrh LeAmps for
my "Music only" system.
So you say construction wasn't too bad? I guess you are
adding the S.E.X upgrades? Why don't you just sell me your
completed and modded Forplay :) LOL
So ya think I will be astounded? I am guessing I will be..
 

Saurav

Senior HTF Member
Joined
Feb 15, 2001
Messages
2,174
Russell,
Yes, the NAD is most certainly solid state.
Brett,
I think I have my mind made up that I will be coupling a Forplay with 4 AudioSource AMP5.3's or 4 nOrh LeAmps for my "Music only" system.
Sounds like a good idea. I need to pick a power amp too, which I'll do once I've decided on my speakers. The Foreplay is capacitor coupled at its output, so you should be able to use the LeAmps with the DC blocking cap removed. I tested this, and it measures 0.00V DC at the output with my Radio Shack autoranging digital multimeter (i.e., not the most accurate in the world).
Just make sure the Foreplay is up to driving the paralleled impedance of those amps. I think it's output impedance is 1300 ohms.
 

Brett DiMichele

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Sep 30, 2001
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3,181
Real Name
Brett
Ooooey Ve...

Okay ya lost me Saurav...

I thought this was going to be "Build it and then plug it

in"

I didn't know I was going to have to do more research into

the amps I choose.. Hurmph..

Can I run 4 SS Amps off this thing? They will be combined

into 2 channels of Stereo Amplification in a Bi Amp Config.

I have not even researched the impedance.
 

Saurav

Senior HTF Member
Joined
Feb 15, 2001
Messages
2,174
Build it and then plug it in
Well, to me, that's totally counter to the whole fun of DIY - tweaking with things, messing with them, changing stuff, trying stuff, that's what this is about. To me, of course. Maybe that's just my nature, I like experimenting. And it's far cheaper to experiment with a soldering iron in my hand, than by buying lots of different components :)
Impedance - rough rule of thumb is, your power amp's input impedance should be at least 10 times your preamp's output impedance. The more the better. The Foreplay's output impedance is (IIRC) 1300 ohms. That means your power amp's input impedance needs to be at least 13 Kohms. Since you're putting two amps in parallel (2 on each channel), the combined impedance is half of each amp's impedance, so each amp needs to be at least 26 Kohms. So, if it's 26K or more, you should be fine, otherwise, you might get weird sound.
This is true with matching any preamp to power amp, not just for the Foreplay. For a "normal" SS pre though, your output impedance may be as low as 200 ohms, so almost all power amps will meet the 10x rule. Tube preamps usually have higher output impedance, so component matching becomes more important. Also, the interconnect cable becomes more important - if you have a long cable with high capacitance, that becomes more of a factor in the overall impedance of the pre-power connection, and can affect the sound. Again, this is true of any pre-power combination, but is more audible with tube preamps because of their higher output impedance.
Chu, I'll take a look. I've heard of that site before, never really checked into it.
 

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