Playing in the Sandbox

[feeench]

Would the Mainline be able to drive the F4?

If you're using a Bottlehead pre amp with an amp, is the voltage swing higher? Or is that limited by the amp?

[Paul Birkeland]

Would the Mainline be able to drive the F4?

The Mainline would be a good choice to drive the F4.   

If you're using a Bottlehead pre amp with an amp, is the voltage swing higher? Or is that limited by the amp?

Unless the input impedance of the amplifier is very low (under 10K), then the voltage swing of the preamp is dependent on the output voltage of the source and the gain of the preamp itself.

[feeench]

So if we assume a constant source voltage, adding a Smash into a chain that goes like this Source --> Smash --> SEX...raises the voltage output at the SEX outputs (Compared to Source --> SEX), right? Due to it seeing more than 2vrms at it's input? If this is true, does this affect the frequency response/distortion levels at the output at the SEX?

How can we figure out the voltage swing of the SEX given a 2v input? This is something I can't seem to figure out properly...

[Paul Joppa]

It takes about 0.62 volts RMS to drive the SEX amp to full power, which is 2 watts. That is 4v at the output for an 8 ohm load.

If you put more voltage into the SEX, it will clip at the output, increasing distortion but not voltage or power. Well, actually, if you put a LOT more voltage in, the output will be square waves and you might get close to 4 watts - but it will all be distortion, not music.

A preamp does not change the amplifiers maximum output capability.

If you had a low-output source of less than 0.62 volts, then that source would not by itself be able to drive the SEX amp to full power, and a preamp could raise the voltage closer to the clipping point (0.62v input to the SEX) and thus get more power and voltage out of the SEX.

[Paul Birkeland]

How can we figure out the voltage swing of the SEX given a 2v input? This is something I can't seem to figure out properly...

You can work backwards by knowing power and gain.  Power into 8 Ohms gives you voltage at the output.  The gain specification will let you calculate what voltage would need to be applied to the input to get that voltage at the output.

If you want to do the calculation without the gain specification, you have to consider the mu of the tubes at their operating points, the step-down ratio of the output transformer and other losses. 

-PB

[feeench]

I have a Smash on the way, and I've got the parts coming for an F4 build. I'm eager to try this out, unfortunately I won't be able to grasp these concepts until I can actually play around with the hardware it seems... But thanks for the explanation you two, it will come in handy down the road.

[mcandmar]

I have a Smash on the way, and I've got the parts coming for an F4 build. I'm eager to try this out, unfortunately I won't be able to grasp these concepts until I can actually play around with the hardware it seems... But thanks for the explanation you two, it will come in handy down the road.

Let us know how the smash works out with the F4.  Then input of the F4 is ~47k ohms so its not really any more difficult to drive then the S.E.X. amp. Voltage level required for normal listening levels are surprisingly low, i have a pair of vintage KEF speakers connected to mine that are easily driven into uncomfortably loud territory with a Quickie.  I measured my Quickie with ~4x gain, 2v rms in, 8v rms out.  Normal listening levels are ~1/3rd volume on the Quickie.  So in short if you have a source outputting ~4v into the F4 it will rock your socks off!

Hope that helps....

Mark

[feeench]

Yeah that helps Mark, thanks. I was a bit nervous at one point there that I might not get the levels I want but I've got a very good idea of what to expect now. I will certainly post results. I'm ordering a set of load resistors for the outputs of the SEX but I'm assuming, since there are no output transformers in the Smash that it's useless to load the outputs there when it's powering an F4 directly?

[feeench]

Okay I got it built.

The Smash with Smash-Up powers it just fine. Got it to go as loud as one would want it to go in a small room with the Orca Minis. I would have to imagine it wouldn't be up to snuff without the Smash-Up.

[JamieMcC]

Playing in the sandbox for beginners 

About a month back for a little project I decided to order the Nelson Pass ACA amp pcb boards from diyaudio.  I put the two mono block amp sections in one diy enclosure and am using my Bottlehead Mainline for preamp and volume control,

Its a little pair of 6W single ended class A, mono block power amps Pass designed to be simple and inexpensive to build for newbies like me. I wanted to try it with some Fostex FX120 speakers but was also curious how it might work with my HE-6 headphones.

Its 6W is sounding good with the speaker set up so I built a little adapter and have been running the HE-6 off of the speaker taps with encouraging  results. I'm going to try some tweaking and swapping out the electrolytic signal capacitors for some film ones next, but this inexpensive little amp I am sure with a little further tweaking is potentially going to make for a really sweet combo for the notoriously difficult to drive HE6 headphones. Its also sounding ok with the HD800 but have a little low level volume noise to chase down with it on them. I am pretty certain its from my unshielded adapter cabling picking up interference (I just used the cabling I had to hand in the parts box).

Its up and running but still requires some feet and a little finishing off plus some experimental tweaking I expect I will swap out the unshielded signal wires for shielded and try a few different film cap options. Any thoughts or tips on optimising and making a little more headphone friendly would be welcome.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fcdn.head-fi.org%2Fd%2Fd2%2F900x900px-LL-d2e8da43_0033.jpeg&hash=4510bffbeda731d157596f380704545c2f942703)
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[debk]

Been playing with the F4 again.  This time with the SR45 (last attemp was with the 2a3 Paramounts).  Had to play with resistor values to find the right combo, using 20R across the SR45 output terminals.  So the chain is BeePre--->SR45-->F4-->speakers.

It sounds greats!.  Just like the SR45 but louder.  Drives my Orca's  better than the SR45 alone, and loses little or none of the SR45 sound with no coloration from the F4.  Really impressed. 

The combo needs 15 to 20 minutes to warm up and sound it's best.  And, as an added bonus for those in colder climates, this combination can probably heat a small room :-)

Debra

[sogo]

The input of the F4 is ~47k ohms so its not really any more difficult to drive..... Voltage level required for normal listening levels are surprisingly low, i have a pair of vintage KEF speakers connected to mine that are easily driven into uncomfortably loud territory with a Quickie.  I measured my Quickie with ~4x gain, 2v rms in, 8v rms out.  Normal listening levels are ~1/3rd volume on the Quickie.  So in short if you have a source outputting ~4v into the F4 it will rock your socks off! Hope that helps.... Mark

Thanks very much for those measurements in and out of your Quickie preamp, and what your volume setting was with the F4 driving your KEF speakers. Levels in the “….. uncomfortably loud territory…” with 2V in and 8V output. Very impressive.

But would you happen to know the model of those KEF speakers, and their actual sensitivity?

And would your listening room size be about 15 ft x 12?

Thanks again for this very valuable info!

 

[mcandmar]

Speakers were KEF Cadenzas, but i am currently in the process of restoring a pair of KEF Reference 104ab's which use the same drivers but with a better crossover design. Looking forward to hearing how they compare. Room is probably around that size, but they would have no problem in a room twice the size.  As for sensitivity the manual states "12.5watts into nominal 8 ohms produces 96dB at one metre and 400 Hz in anechoic conditions" so yeah, not sure what that tells you

I am also experimenting with 6A3 single ended amps and have been very surprised how well they drive them with a few watts.  The F4 is more controlled in the low end which i assume is due to the better damping, but i don't find it wanting for power as long as you keep the volume levels sensible.

[sogo]

Been playing with the F4 again.  This time with the SR45 (last attemp was with the 2a3 Paramounts).  Had to play with resistor values to find the right combo, using 20R across the SR45 output terminals.  So the chain is BeePre--->SR45-->F4-->speakers.

It sounds greats!.  Just like the SR45 but louder.  Drives my Orca's  better than the SR45 alone, and loses little or none of the SR45 sound with no coloration from the F4.  Really impressed. 

The combo needs 15 to 20 minutes to warm up and sound it's best.  And, as an added bonus for those in colder climates, this combination can probably heat a small room :-)
Debra

About your comments about your F4 and tube integrated heating up the room, you might find the following experiment very interesting, unless you already did it.

But first what is the sensitivity of your speakers? At least 89 to 90db?

And do you play them not much more than ~ 75db at 70Hz and 53db at ~7kHz at 11 ft away, as per the revised Fletcher-Munson equal loudness curves? See the curves in the wiki report.   

If yes to both questions, try this sound quality vs. power consumption experiment: Turn down the bias on the F4 so that it stays in Class A for the first 10 to 12 watts/channel.

No doubt the chassis will feel a lot cooler. But will the sound quality suffer a lot? Again if you answered yes to both questions above, if you use one or two powered subs from about 70Hz on down and if your room’s not too big then you might be pleasantly surprised on both counts. That is, the slightly underbiased F4 amp will still be able to stay in Class A and will be running a nice bit cooler too.

One thing: You said that the combo needs 15 to 20 minutes to warm up to sound their best. That’s less than half the preferred warm up time stated in the F4 manual.

Did you build your F4 clone to do 25 wpc into 8 ohms and 40 wpc into 4 ohms, like a stock F4 amp does? If it puts out a lot less power than that then maybe you can’t do this experiment. Otherwise, why not give it a shot?

Btw, is this that "Simple 45" amp that you built to drive the F4 amp the same one that was reviewed at dagogo.com? Sorry, not allowed to post the link. 
 


 

[sogo]

Speakers were KEF Cadenzas, but i am currently in the process of restoring a pair of KEF Reference 104ab's which use the same drivers but with a better crossover design. Looking forward to hearing how they compare. Room is probably around that size, but they would have no problem in a room twice the size.  As for sensitivity the manual states "12.5watts into nominal 8 ohms produces 96dB at one metre and 400 Hz in anechoic conditions" so yeah, not sure what that tells you....The F4 is more controlled in the low end which i assume is due to the better damping, but i don't find it wanting for power as long as you keep the volume levels sensible.

According to Pat D at audioreview.com's forum, the circa 1974 (?) Cadenza is 83db/1watt/meter. That’s not considered a sensitive speaker compared to Lowther, Azurahorn, Bastani, Audio Note and countless others in between. DebK’s Orca Deluxe that I just read about are 89db. That 6db difference quadruples the apparent loudness per meter compared to your 83db Candenzas, and for the same amount of applied power.

But the fact that the F4 can drive the Candenzas into the “…..uncomfortably loud territory…” in your room size with 2V in and 8V output applied to the F4 is excellent news. And that’s why I suggested that experiment to DebK. I’m in no position to guarantee anything, but it seems plausible that unless she ever intends to really crank up the Orcas volume (and her powered subs), that the bias on the F4 can be tapped down a bit to have a somewhat cooler amp-so long as there’s no perceived loss of sound quality (i.e. soundstage, depth, tonality, clarity, detail, imaging, micro and macrodynamics). Would you consider trying this?