[jrebman]

Anand,

Now that's what I call a project list .

Tom, very timely as just a week or so ago I got a set of boards from Peter D. One amp board and two PS ones -- dual mono supplies are what I've been thinking about since day 1 on this project.

It's going to be a good while before I get to it though, and the dual mono octal Aikido to pair with it (6sn7/6h30pi). I'm considering adding an extra pair of output FETs so that I can get a reasonable level of power into the Sapphires -- probably 100 W or so into the 6 ohm load.  The Conrads seem like they'd be up to the job.


Definitely keep us posted.

-- Jim

[TomS]

Anand,

Now that's what I call a project list .

Tom, very timely as just a week or so ago I got a set of boards from Peter D. One amp board and two PS ones -- dual mono supplies are what I've been thinking about since day 1 on this project.

It's going to be a good while before I get to it though, and the dual mono octal Aikido to pair with it (6sn7/6h30pi). I'm considering adding an extra pair of output FETs so that I can get a reasonable level of power into the Sapphires -- probably 100 W or so into the 6 ohm load.  The Conrads seem like they'd be up to the job.


Definitely keep us posted.

-- Jim

Jim,

I suspect the F4 will drive the Sapphires just fine, especially with the dual mono supply.    Honestly, I probably wouldn't mess too much with the design by adding the extra transistors.  You might be better off to consider a second monoblock setup in that situation. 

For what it's worth, I hooked it up to my Merlin VSM-MX's tonight and it drove them beautifully.  I think they are just very compatible with the low damping factor of the F4.  The honesty of "tone" on acoustic instruments seems just right.

Good luck!

Tom

[jrebman]

Tom,

What's the sensitivity and load characteristics of the Merlins like?  I figure the stock amp at suggested bias could probably do 75 watts, give or take into the Sapphires.  Seeing that they do alright with 28 watts of tube power, I'd guess that would be fine.

-- Jim

[poseidonsvoice]

Jim,

The Merlins I believe are 88-89 dB/1w/1m. About an 8 ohm load. It peaks at about 20+ ohms near the crossover point and lowest is around 6 ohms. Bobby Palkovic has the Scan Speak 8545 woofer custom made (with probably a larger magnet) to bring up the sensitivity of the unit. Most loudspeakers that use the Scan Speak 8545 are usually 86 dB/1w/1m sensitive. No big deal really. The F4's should drive them fine except in a concert hall.

Best,
Anand.

[TomS]

Yes, they're 89 dB, 1W/1M (2.83V).  They do use the SBAM equalizer which adds 5.2dB of boost at 35 Hz (takes a little power).  Since they work so well with the Bella Extreme and most little push pull tube amps I expected them to friendly with the F4 and they definitely are.  Plenty of output in my 24x13x8 room too.  I thought the Sapphires used a similar, though stock, version of that same woofer.

[poseidonsvoice]

Depending on which version of the ACI Sapphire you are talking about (I've known this design since they used a Focal Kevlar midwoofer), the sensitivity is a little different. The stock Sapphire XL is about 85.5 dB sensitive and is a 6 ohm loudspeaker (with 4.5 ohm as the low). In addition it uses the Scan Speak 5.5 inch Revelator woofer which is very different from the Scan Speak 8545. Even still, the F4 should drive it fine.

Best,
Anand.

[jrebman]

Can I ask how much filter cap capacity you have in each PSU of the dual mono?  Would you think 30,000 uF enough for a monoblock?  I thought there were some concerns about the max current handling of the Jensens.

I have lots of notes on the F4 on my dead computer, and as soon as my new one gets here, I'll go retrieve all those files, and then I won't ask so many stupid questions .

-- Jim

[jrebman]

Anand,

Sorry, they are indeed the XLs with the Revelator woofer... and I love them

-- Jim

[TomS]

Can I ask how much filter cap capacity you have in each PSU of the dual mono?  Would you think 30,000 uF enough for a monoblock?  I thought there were some concerns about the max current handling of the Jensens.

I have lots of notes on the F4 on my dead computer, and as soon as my new one gets here, I'll go retrieve all those files, and then I won't ask so many stupid questions .

-- Jim

I used four 15,000uf 35v Panasonic TSHA caps (Digikey P6668-ND) on each side, eight total for the stereo amp with dual mono supplies.  Peter's PS board is a CRC with 15,000uf C, four x 0.47R 5w Mills resistors, then 15,000uf C on the load side.  You could fit the 22,000uf ones in if you want.  Some people definitely had problems with the Jensens due to current load.  I personally think they're overkill, but that's half the fun to try it and find out if it makes your ears happier.

One important thing with Peter's board design I wasn't too keen on is he uses the expensive Vishay S102's, among others, that have very narrow 0.2" lead spacing.  I just used 1/2W PRP's and stood them all on end.  It doesn't look great but it's convenient for clip leads to measure things.  Also, the 5w source resistors and power supply resistors must be spaced off the PCB to let get air around them.

[poseidonsvoice]

Can I ask how much filter cap capacity you have in each PSU of the dual mono?  Would you think 30,000 uF enough for a monoblock?  I thought there were some concerns about the max current handling of the Jensens.

I have lots of notes on the F4 on my dead computer, and as soon as my new one gets here, I'll go retrieve all those files, and then I won't ask so many stupid questions .

-- Jim

Well, its definitely an experiment. Looking at my F4 schematic, it looks like N.P. recommends 60,000 uf per voltage rail. Using Peter Daniels boards,this is definitely possible, using 22,000 uf  Jensen 4 poles or moving up to the 33,000 uf Jensen 4 poles (40 V types). I e-mailed Peter a while back about the max current issue somebody had brought up on diyaudio and he didn't have a very good explanation. The 4 poles are expensive so he recommended Nichicons I think. This particular F4 I was planning on using for the satellite section of my speaker so I may not need all that capacitance. I think I will go with something in between like 22,000 uf for each cap, or about 44,000 uf per channel.

My other F4 would use just Panasonic TSHA's as Tom has recommended. This one would be a true dual mono, with two separate boards for each channel as he has done. So here I use four of the 15,000 uf TSHA caps per channel or a total of eight as he has done.

Best,
Anand.

[Christopher Witmer]

Perhaps you already were thinking this, but one cool thing about the F4 is you can run the speaker output of the 845 directly into the F4 and take advantage of the added current output it provides while retaining the sonics you like about the 845 amp.  This thing is incredibly versatile if you think outside the box a bit.  Nelson Pass truly went a different road just by releasing this to the market IMHO.

Very interesting indeed! I'm not at all well informed about this sort of approach; has anyone already done it? And can anyone point me in the right direction to learn more?

Thanks!

-- Chris

[TomS]

Take a look at the First Watt website.  Here is the F4 manual that explains all the options.  There is also an extensive review on 6moons.com

http://www.firstwatt.com/downloads/f4_om.pdf

[TomS]

Just a little update.  I finally built the second F4 up to enable balanced 100w Class A monoblocks.   Since the Conrad heatsinks work so well, I was able to bias them a bit higher than before (about 0.6A per device).  Nice and toasty for the winter months.

Now on to the Pumpkin/Shunty balanced preamp project to swing the F4's to full output and finish up an F5 for the top end of my CS2's.

[zybar]

Great job Tom.

Let us know how they sound when you finish the other pieces.

George

[gbeard]

Very nice Tom! 

I was hoping to build a single stereo unit this winter and have the majority of the parts, but new job responsibilities have kept me from starting. I hope you'll give me some pointers!

Cheers from Bloomington,
Gary

[TomS]

Very nice Tom! 

I was hoping to build a single stereo unit this winter and have the majority of the parts, but new job responsibilities have kept me from starting. I hope you'll give me some pointers!

Cheers from Bloomington,
Gary

Gary,

Glad to help in any way I can.  The F4 really goes together quite easily and is straightforward to get up and running.  The real trick is to provide enough grunt (voltage) to drive it if you don't have 100db+ horns.  Best of luck!

Tom

[gbeard]

Thanks. I am very grateful to have a good job but it gets in the way of all the fun stuff!  I still need to upgrade my AudioTropic Moebius to the current specs too. I had all these wonderful DIY plans, but now I am commuting south to the river for a two year stint.

Maybe once I get my accomidations figured out I can bring my gear down and do some serious welding...

G

[JoshK]

An aikido preamp would likely work well for driving the F4's, but I'd use a 6SL7 or 12ax7 upfront and keep the cables really short. 

[TomS]

An aikido preamp would likely work well for driving the F4's, but I'd use a 6SL7 or 12ax7 upfront and keep the cables really short. 

I have an Aikido but it is single ended and I'd like to run them as balanced mono's.  John B sent me a few details on his balanced version but I'm still waiting for more info from him.  Unfortunately it needs both +/-300vdc so won't be the easiest to implement.

[poseidonsvoice]

I bet my F4's would work nicely with my future dream speaker - the Geddes Abbey. Nice job Tom!

Anand.