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PostPosted: Sun May 13, 2007 8:49 pm 
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I was considering biamping my Newform Research R630 speakers. I have a Forte model 3 and a Threshold 400a. I could use the Forte for the midrange and the Threshold for the ribbons.

I may also purchase a Threshold sa/3 for the ribbons and use the Threshold 400a for the midrange. Which choice would be better? I have a Threshold t-3 preamp coming and a Marchand xm-9 3 way crossover (I also have an Axiom ep500 sub for the bass). This system is used for both stereo and home theater (with a Theater Master Ovation processor).


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PostPosted: Sun May 13, 2007 10:27 pm 
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Unless your Threshold 400a has had a rebuild it will probably sound a little darker than your Forte and the sa3. So I would probably use the forte and sa3 for mid and top end work. Which ever sounds better in which position your ear will tell you.
Your 400 a is a great amp and would really change up with it's electrolitics done.
I've owned a few 400a over the years and all but one needed to be updated because the heat does cook [ dry out ] the old caps.


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PostPosted: Mon May 14, 2007 2:28 pm 
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Yes, the Threshold 400a has been updated with Blackgate caps, new diode rectifiers, several new output transistors and Cardas binding posts. This was done by the previous owner and DTSaudio (Dan) and I think he did a great job.

I found that it sounds a little warmer than the Forte model 3, which has not been modded yet (but will be in the future). The Forte still sounds very nice but it sounds a little less transparent when played loud, even though its rated at 200 w/ch. Class A amps just seem to have the same sound characteristics no matter how loud I play them.

I was thinking that a modded SA/3 might sound even more transparent and detailed in the high end than the 400a, that's why I was thinking of using it for the ribbons and the Threshold 400a for the midrange. But I can try both ways I guess (I haven't bought the SA/3 yet). I really like my Threshold amps compared to my bridged Bryston 2b Lp's that I was using before, they sounded too bright and hard (electronic) with my Newforms.


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PostPosted: Mon May 14, 2007 3:39 pm 
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If you can pick up the SA-3 for a good price I would jump on it.
Both the SA-3 + 400a are class A amps. The Forte 3 being class A/B.

The newer version of the model 3 was the Model 6 and the Model #6 can be Modded to run in pure Class A, as the Model 4 pure Class A and #6 are pretty much the same amp. So converting it is not to much work at all.

I am not sure if your Model 3 can be converted to Class A.
But you can ask Jon Soderberg at http://www.vintageamprepair.net/ if it can be done.

Jon worked at Threshold with Nelson Pass and does upgrades, rebuilds and Modds to Threshold + Forte amps.

A year os so ago the convertion from the Model#6 to Model#4 was about $150. US plus shipping and if you have some skills, Jon will even tell you how to do it. This is the way I went and so saved on shipping costs, as there wasn't much to the Mod.

The Class A version of "my amp" is far, far better the the class A/B version, easy. even though the A/B version had 4x as much watts.

Many think the SA series are the best of Nelson Pass, even better then the newer Pass Labs, some will say.

I've used Threshold and Fotre amps with a few different Planner speakers.

Maggies and Eminent Technology LFT 8A's with great results.

You can't go wrong with Threshold of Forte. Great Amps!!!


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PostPosted: Mon May 14, 2007 6:32 pm 
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I never thought of modding my Forte model 3 to class A mode. If I can't get the SA/3, I'll consider modding the Forte model 3 to class A. The class A version was the Forte model 1a, though. The model 3 has bipolar output transistors whereas the model 4a had the IGBT transistors. I don't think that it would be that easy to swap out all of the output transistors. I might be better off looking for an original class A amp (Forte model 4a or Threshold SA series) and upgrading it with better caps, diodes etc. At the price you quoted, that is quite a bargain!

I think that I will still use the Forte model 3 in a second system that has Monitor Audio Gold MA700 speakers. I am using a Bryston 2B LP in that system and it sounds a little dry but clear. The Forte is a more sweet sounding amp with more air, it should open up the Monitor Audio speakers a bit. I am using a M&K mx70 sub with these speakers, so bass is no problem. I love these old Threshold amps, I think that they are a real bargain at todays prices, even with the cost of upgrading.


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PostPosted: Tue May 15, 2007 12:14 am 
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The 1a is the Class A version of the Model #3 and what I meant by my talk of converting, is that maybe the Model 3 can be converted to the Model 1a. Sorry if I wasn't clear on that.
The Model 3 may be very much like the model 1a and maybe can be converted easily. Jon Soderbreg would know.

Quote " I might be better off looking for an original class A amp (Forte model 4a or Threshold SA series) and upgrading it with better caps, diodes etc. At the price you quoted, that is quite a bargain! "

The price I quoted is just for the convertion from Class A/B to Class A.
But if one was to send thier amp down for serivce, it certainly would be a great time for some upgrades.

I also have a few S/150, series I and II. The series II had just gotten an upgrade by Jon Soderberg before I bought it, with new main caps at double the size as the org. ones (same size caps as S/300), with bi-pass caps on the main caps as well, diodes etc. The price for this 2+ years ago $375. US + shipping. Alot of bang for the $$$ with Jon Soderberg.

I got all the paper work with the amp when I got it. I also added Edison price pure copper binding posts, Vampire copper gold plated RCAs, Furutech IEC insert and Acme silver plated fuse holder for the main AC.

It's to bad I have to many amps right now and I'll will be selling one
But which one is the question. I have 4 amps and only need 2.
At one time I was going to do a 3 way active setup with some Maggies so I had been getting some gear together.

But I'll be doing a 2 way active setup with my Eminent Technology LFT 8A's using a Marchand XM-44 x-over.
The reason I havn't done it yet is, well I just lazy!!!
Gotta get off my ass and get it done.

You certainly are right, these amps on the used market are a great bargain.

If only they made the Armored Personal Carriers as Bullet proof as Threshold amps!! lol

PS. Welcome to Cam Albert!


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PostPosted: Tue May 15, 2007 9:09 pm 
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My mistake; I should have read your post more closely. I think I might pass on the SA/3 after all and try to save up for SA/1 mono blocks. They would be easier to biamp than two different amps. They are also supposed to be a little better than the Threshold 400a as they have separate power supplies and probably larger supply caps. There is a pair of SA/2e's right now on Audiogon that I might look into. They are said to have been upgraded lately. I just wonder if 100W is enough for me as I find the Threshold 400a to be just a little bit low powered for my Newform speakers (only on rock music - the amp is more than enough for classical or jazz). Perhaps I'm getting a little carried away with volume; I just find it amazing how clear the amp can play loudly with no compression at all.

Thank you for the excellent advice. I have only had the Forte and Threshold amps for about a month now and you have much more experience than I do with the Threshold gear. I also have a Forte model 2 preamp and have a Threshold T-3 preamp coming; I have definitely become a Nelson Pass fan!


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PostPosted: Sat May 19, 2007 12:31 am 
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You will have a lot of more power when you go active.
The inductors and caps in a passive x-over suck up alot of power, once they are gone, your drivers will see all the power from your amp.
So I wouldn't worry about not having enough.
I know a few on the Plannar form on Audio Asylum who run the III series Maggies with active x-overs and some will go with 100 watts on the Bass, 50 watts on the midrange, and 25 watts on the Tweeter (ribbon).
You don't need as much power when you go up into the midrange + tweeter, they are much easier to push and using an active x-over makes it even more so.

What are you doing with the bass?

SA/1 mono blocks or the 400 would be overkill power wise for the tweeters. Your first thoughts of the SA/3, I think is a great way to go.
But it's your money! I just want to spend it. :lol: :lol:


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PostPosted: Sat May 19, 2007 8:41 am 
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From my experience,
Rather than horizontally bi-amping (one amp for the tweeters - one amp for the mid / bass),
I would suggest vertical bi-amping (one amp for the left and one for the right speaker)

Unless the Forte / Threshold power amps are built like dual mono blocks, with excellent channel separation, I would suggest vertical bi-amping.
This method will really open up the sound stage.


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PostPosted: Sat May 19, 2007 8:44 am 
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I am using an Axiom EP500 version 2 subwoofer for the bass, crossed over at 40 hz. It is a 500 watt sub that is DSP controlled to within +-3db to 20 hz. I may have given HSU or SVS a try but the shipping and broke(rage) fees would have been too much. The Axiom turned out to be a nice sub that sounds clean and fits in smoothly with my Newform's.

As I have been upgrading quite a bit lately ($$$) with 2 new preamps (Forte 2 and Threshold T-3), 2 amps (Forte 3 and Threshold 400a), Marchand xm-9 crossover and a sub, I think I'll try the Forte model 3 on the midrange and the Threshold 400a on the ribbons first. If I turn the gain down 3db on the crossover for each amp, I should be at a safe power level as the bass is rated at 100w and the ribbon at 50w. I am just waiting for the frequency modules from Marchand and then I'll give it a try.

I was thinking of the SA/1 monoblocks because I have heard that they sound better than the 400a. My room accoustics and cheap cables probably make a bigger difference, though. I guess I'll be spending even more money (eventually) in that area too. :shock:


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PostPosted: Sat May 19, 2007 8:53 am 
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I have useed vertical biamping in the past with two bridged Bryston 2B LP's (200w/ch bridged) and it sounded very nice but a little bit bright and cold. The Bryston amps sounded a little bit harder or more SS than the Thresholds did (either the Forte 3 or Threshold 400a). I prefer the warmer, tubelike sound of the Threshold amps and they still seem to have a nice soundstage with my speakers (Newform Research R630). Ribbon speakers have in general a really wide soundstage.

I think your right that mono blocks would be a better way to go, that's why I was considering getting a pair of SA/1's in the future. Maybe I need a second job. :roll:


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PostPosted: Sat May 19, 2007 10:27 am 
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Actually, I have a Forte model 2 preamp.

Its a Classe A design using both discretes and Op Amps.

I think the original OpAmps are Linear Technology's LT1028's.
These are in sockets, so they can be replaced very easily with newer technology OpAmps.

I've heard the Analog Devices OpAmp AD 744 in the model 2 and I would not recommend these.

However, the Burr Brown OPA 627 is considered to be one of the best Op Amps going.
I've got a pair of these on order for the model 2.

If you change the Op Amps, I would recommend supporting the circuit board from the back while plugging them in.


Also, I think there were 2 versions of the outboard power supply for the Forte model 2.
One looked like a wallwart and the other is in a flat metal case.
My model 2 has the flat metal case.
Openning this up, I am quite sure this can be improved.
As Nelson Pass said, one of the 3 ways of minimizing hiss, on a preamp, is to minimize the hiss on the power supply voltage rails.
.


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PostPosted: Sat May 19, 2007 4:18 pm 
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My Forte 2 has the slim case power supply and probably the original OpAmps. I think that I will replace the old OpAmps with the Burr Brown OPA 627 ones like you recommend. This would probably increase the dynamics and further reduce the noise of the preamp.

I hadn't looked inside my preamp; its nice to know that the OpAmps can be upgraded so easily. I still like my Forte 2 and plan on keeping it so upgrading is a good option. I will also upgrade the caps to Blackgate ones in the future though I will probably do that with the Forte 3 amp first.


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PostPosted: Sun May 20, 2007 5:06 am 
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The data sheets for the LT1028 and OPA627
show that the LT1028 actually has a better noise figure than the OPA627.

I would suggest holding off on buying OPA627's until I get a chance to review them in the Model 2.

I'll let you know within days after I had a chance to review the OPA627's in the Model 2.


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PostPosted: Sun May 20, 2007 8:50 am 
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Thank you for checking on the OpAmps for me. I look forward to your review.


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