New amp at home: Forte Audio 1A

[tpg]

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[tpg]

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[Speedball]

...really nice amp, I would like to try something like that with my Rf7's...[]

[Marvel]

tpg,

So, does your mom understand now, that there was something wrong with it?

Looks nice. His designs are great.

Bruce

[mikebse2a3]

Hey tpg

That looks like a very well built amp!!! I really like the layout.

I see its also dual mono.

Just curious but do you happen to know what roughly are these selling for on ebay and such?

mike[]

[tpg]

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[leok]

Nice looking amp, and, I'm sure, nice sounding. how are you driving the

amp now (you said you're looking for a pre-amp)? Have you tried a

simple attenuator? If you do add a pre-amp I'd be interested in your

impressions.

Leo

[mikebse2a3]

Nice looking amp, and, I'm sure, nice sounding. how are you driving the amp now (you said you're looking for a pre-amp)? Have you tried a simple attenuator? If you do add a pre-amp I'd be interested in your impressions.

Leo

Thats a good idea Leo.

tpg

If your system is simple ( not many sources ) then an attenuator could be a very good choice for sound and cost. I used a Creek OBH10 for many years and it sure sounded better than alot of pre-amps (some selling for as much as $2K.

mike[]

[mobile homeless]

Why would you cut the bias way down? Amps biased high in Class A run hot - seems cutting the bias "way back" would take away some of the very properties that make it desirable (although where is the recommended setting?). I had some B&K Sonata M-200 Monoblocks that benefitted from increasing the bias higher into class A at the factory. I wouldnt be worried about the amp running hotter (One interesting point, it's one if the few SS amps I have seen that is apparently easily user adjustable for bias). But If you've felt a Class A amp before, they usually run far hotter (extra ventilation space is helpful). I've noticed this in every Class A SS amp I've used. I've never heard the Forte 1a but have heard the Forte 4a. It's also a 50Wpc Class A amp that runs very hot and sounds extremely nice for SS. It's another bargain on the used marget.

On preamps, I have had a handful of passives and still own a Creek OBH-12. Excellent transparency for the ducats although I think a quality tube active stage is an excellent combination with a SS amp like you now own. In my view, you get more midrange warmth, drive, and a certain harmonic richness that is a good compromise. I like passives but always find myself going be to a good tube "active line stage" preamp, even if all the qualifications for a passive are met. On the other hand, if your system/setup meets all the requirements, a good passive is the cheapest way to this level of transparency (the transformer passives are the latest passive rage).

kh

[tpg]

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[PrestonTom]

TPG,

Congratulations on your new amp.

1. re: changing the bias - unless you really know what you are doing (and frankly, most folks would not), I would receommend NOT changing the bias - leave it at the factory spec.

2. re: "Passive" pre-amps - these are certainly an option if you are not using a phono. Please keep in mind two issues. The first is the relative input & output impedances of the various devices. The output of the CD should be relatively low and the input of the amp should be relatively high. If this is the case, then a well designed passive pre-amp will do fine. Second, the interconnect cable from the passive to the amp should be as short as possible. There will be a small capacitance in the interconnect. This capacitance increases with length. If there is too much capacitance, then you will inadvertantly create a low pass filter (1 / 2*pi*r*c) that may roll into the audio spectrum. This is simple physics and not "cable voodoo". Many are unaware of the problem however.

3. Possibility for a passive pre-amp: You can either build one yourself using an audio pot, or stepped attenuator, or transformer (some careful thinking is required about the impedance issues however). My suggestion would be to go check e-bay for an Adcom SLC 505 (they called it a straight line controller). They are no longer made, but you can get them for $50-100. They are well-built.

IMHO its criminal how much some of these high-end audio companies charge for equivalent circuit.

Good luck,

-Tom

[leok]

tpg,

I don't know the input topology of your specific amp, but the inputs on

some N. Pass designs I've seen don't present a simple resistive load

(as would a summing junction style input). This could, as you have

mentioned, introduce nonlinearity with a high impedance input

(resistive in the case of attenuator or something more complex in the

case of a tube pre.

You can purchase simple audio attenuators at pro and semi-pro

electronic supply outlets. If you can't find one, I have one here I'll

mail to you. I was using it with the SET system until I built the 4

channel DACT resistor network divider. It's 50KOhm and gives you a

resonable idea of what a passive attenuator can sound like. The

DACT-based attenuator does sound cleaner, more detailed, and quieter,

but the cheap one told me where the ballpark was.

Leo

I took a look at a few NPass DIY designs. The inputs tend to be FET

diffamp configurations with resistors to reference ground. The ss

nonlinearities are pretty much out of the picture .. so the low

impedance can be considered simple linear resistance and attenuator

should work. Again, I haven't seen your exact circuit. Anyway,

attenuators are cheap and easy to try and keep around (as Kelly does)

for comparison and those occasions when they make sense.

[Erik Mandaville]

tpg:

If you can offer something about your budget for an active preamp (if that's what you want), some suggestions could be made that might be worth considering.

Most CD players have the ability to drive many amplifiers very well, and some well into the clipping range, which is why (as Leo and others pointed out) it's possible to often get by quite well, if even on just a temporary basis, with a volume control between the CDP and amp. What I have also found useful is a dual-mono passive, using two mono controls rather than a stereo potentiometer. If there is some channel balancing problems they may be present, having the means to independtly control each channel can be helpful.

My two channel amplifier (which also serves as the rear channel amp in our surround system), a MOTHSi2A3, is an integrated stereo amplifier with a 'passive' volume control on board. It works fine that way, although I truly prefer what my own 6SN7 preamp adds to the overall sound. I've gone back and forth from using just the control on the amp, to using the active line stage preamp, and have been keeping the active preamp in the system on a permanent basis. The Moth is thus used with the VC virtually wide open. I may simply remove the VC and in it place install a 100K ohm resistor. The active stage seems to provide a liveness or sense of speed and immediacy that is maybe not quite so evident, with this particular amp, with just a passive atttenuator. But this is my system, and what works well for you may be something altogether different.

As Leo mentioned, if you're handy, a passive is a great little project, and takes very few parts. It would give you an idea if it's something that you could be happy with. I know the Creeks are made well, and my brother really likes his, but you can make something that will perform very similarly at a substantial savings. As Tom said, long runs of interconnect just don't work well for this. High output impedances have difficulty driving the capacitive loads of long cables. I wanted to mention, too, that just because an active line stage happens to be vacuum tube-based (I think this was mentioned above), it doesn't necessarily mean that the component will have a high output impedance. In fact, some tube preamps have very low output impedances. I really agree that a good tube line stage might make for a great combo with this amp, and there are good designs available that don't cost a fortune. Until then, a decent passive -- home-made or store bought (which may not be too much cheaper than some active designs) might be perfect.

Erik

tpg: I have to say I also really agree with the bias adjustment issue. I would follow the advice above on that and not fool with it unless you are very sure about what you're doing. There are some pretty hefty capacitors in there, and those things can shock very hard, even if the amp is unplugged. Be careful.

[TheSoundBroker]

Oh man, that brings back some memories! I used to sell Forte when I

managed a high end shop in Santa Barbara. The F-4 was one of my

all time favorites and still remains so...such a sweet amp! That along

with the Linn Klout (a STUPIDLY good amp!) made up the bulk of my

amplifier sales.

Do NOT turn the bias down...it is what makes that amp so sweet!

[tpg]

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[Erik Mandaville]

tpg:

"I do build amps and such fairly often, so a passive pre would be no real problem. And, I am pretty well versed in Pass amps, and am very careful when inside of a live amp. "

I didn't realize that. I just mention the safety issue out of habit, or something......

In any event, I understand the money issue well. In that case, a simple attenuator might be great for right now. Radio shack has everything you need, although some of their prices for small parts are pretty crazy.

I see what you're doing as far as the bias control. Good luck with that.

Erik

[tpg]

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[Marvel]

Well, I found a nice looking design for a passive

preamp. Built it up using parts I had on hand... and fried the

left channel of my computer's soundcard output. Apparently the

design is flawed in its method of shorting signal to ground in the

voltage divider... (if you haven't already guessed, this has not

exactly been my week)

So, I am back to the drawing board

again. I missed my chance to go to the electronics store because

the guy has closed now. So, RadioShack is it. OH!

I'll be near a RadioShack tonight so I may drop in and pick up a 100k

pot. It would be better than nothing...

That would depend on your sound card. If it is an SB, they are

notoriously fragile. A bummer anyway. I had to feed my soundcard (one

of them anyway) into a line level mixer, just to buffer it to feed the

inputs on my ST-70. Otherwise it sounded awful.

Bruce

[tpg]

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[leok]

tpg,

The "attenuators" I was refering to, that can be obtained at

electronics stores (the type of store that caters more to the

electronics professional .. such stores might also sell test equipment,

tools, PA devices, etc) are ready to use devices with appropriate male

and female RCA connectors. You don't have to build one from scratch.

They're not expensive and like I said, you can borrow mine if you like

.. just contact me via message or email through this site.

Leo