BK ST-140 pre-amp

[newindustar]

I just came home with an old BK-140 amp (not 140M) and an Onkyo P3200 preamp and matching T-4500 R1 tuner.

I had asked the question if normal preamps like the Onkyowere compatible with the BK but I answered that myself and hooked it up so the preamp part of this question is answered.
I have the BK running off the Onkyo now. sounds great.

I do have another question.

Before I got the BK I had two pairs of 50 watt 8 ohm speakers, (Nova-15 and Utah 3-way 12in acoustic suspension) running on my Pioneer 6500II. These blended very nicely running one pair on A and the other pair on B. I considered this two channel, A channel and B channel which required 8 wires.

The BK is a two channel but only has 4 speaker wire connection posts.

I am new to this so can you explain the discrepancy here? I would like to run my two pairs of speakers off the BK.

Can I hang these two speaker pairs from each side the BK? If so would they be in series or parallel?

 

[Bimphert]

Yeah, I own a couple of ST-140's.... Great little amps.... Regarding your 2nd Q. I wouldn't run 4 speakers from this amp. If you wanna run all 4, get another ST-140.

If you wire the spkrs as you suggested, you'd present a 16ohm load (wired in series) and decrease the rated power (amp) by 1/2. If you wire them in parallel, you'll create a 4ohm load (doubling the amps power) but these amps don't respond well to sustained low impedances (especially if you have one of the earlier models with only 2 output transistors per CH).

If your speakers were 4ohm (instead of 8) you could likely wire them in series (to create an 8ohm load total). But that's not the case.

ST-140's are fairly inexpensive ($50-$100) on ebay. If you love your speakers that much, buy another ST-140.



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[thedelihaus]

If you want to run more than a single pair of speakers, you can get a speaker selector switch- these are made by Niles, Adcom made some, as does Sima, Monster, etc...

These protect your amp from difficult loads and is healthier for your amp.

Running one pair on A and the other pair on B is indeed two channel, A channel and B channel, which require 8 wires, but presents a 4 ohm load, but can dip below this- not particularly good for many/most amps.

And running two pair of speakers can also sound good to a point/in general, but can also introduce issues of odd phase coherence, funky response, imaging, soundstage, etc.

The B&K ST-140 is a great amp. I owned the ST-202, 202+, and currently own the EX-442 (two of them) and EX-442 Sonata- and really love them.

My thought is look for one air of speaker that does what your current two pair du, but better.

 

[newindustar]

Work intervened -

The series parallel issue was I thought it would be from what I remembered form electronics classes. The good thing is that BK changed the dynamics of how my speaker sound on the lower watt receivers and amps I have including a Sony STR-7055. The Realistic Nova-15s very clear and bright, on the high side, really nice but gained more depth on the BK/Onkyo pre. What really changed was the old Utahs. They were a bit muffled and boomy and lacking clarity on the less wattage boxes but responded with much greater range on the BK. They play really together. I don't know how much the Onkyo preamp has to do with it because I can't isolate the pre from the BK. Anyway I am very pleased just running the Utahs. At about 1/3 preamp volume they really gets some heart. I took the Novas out as well as the equaliser. The Utah's have an adjustable crossover. I read they are some thing like old Advent or ARs in terms of being sealed acoustic suspension and alnico. Not expert on that but it works now where it really wasn't in my original config. I also lucked in to a 500.00 range Onkyo DVD/CD changer from around 2000 so now have a nice 3 layer Onkyo stack. I am pretty set right now, best I ever was able to put together and incredible from thrifts. Playing Ronee Blakely on vinyl right now, sounds so good. Does an amp like this just have a different dynamic compared to smaller watt amps/receivers? it is about watts are is there a different voice due to design? I konw some people say amps do not color, others say they do.

Regarding just what a channel is, I googled around and it was not so clear. Generally it seems left is one channel, right the second. I was sort assuming a and b were channels as said above. I could see a or b or c ( my Sony has all 3) as sending out 2 lr channels to different speaker pairs say in different rooms one at a time. I don't quite see what is happening with a+b however. How does the amp/rec maintain the impedance at 8 ohms at each speaker in a+b?. If channel is left right running two pairs would seem to running in parallel i.e. 4 ohm per speaker. How is the power divided.

 

[Bimphert]

Work intervened -

Regarding just what a channel is, I googled around and it was not so clear. Generally it seems left is one channel, right the second. I was sort assuming a and b were channels as said above. I could see a or b or c ( my Sony has all 3) as sending out 2 lr channels to different speaker pairs say in different rooms one at a time. I don't quite see what is happening with a+b however. How does the amp/rec maintain the impedance at 8 ohms at each speaker in a+b?. If channel is left right running two pairs would seem to running in parallel i.e. 4 ohm per speaker. How is the power divided.

Correct! Left is USUALLY one channel and right is the other.

I know what you mean by the A + B channels (same thing on my old Onkyo 506RS & Yamaha M-70). Usually, A-Left and B-Left represents one CH, A-Right & B-Right represent the second CH. It clearly states in my M-70's OM that EITHER A OR B may be connected but NOT both at the same time! REMEMBER, there are still only TWO channels but some people think they in fact have 4.They are wrong! I can't tell you how many people I now who fried their amp because they had 4 X 4ohm speakers wired and pushed the amp beyond its limits. Some Amps can handle low impedances but most cannot. The B&K ST-140 cannot (especially if it's one of the 1st 3 models in the 140 line-up). I think there were 4 or 5 variations of the ST-140 in all but can't say for sure. There were @ least 4 anyway! The first two variations of the ST-140 had 2 outputs/CH. BK eventually changed the layout to incorporate 4 outputs/CH and these amps are better suited for lower impedances. They also had an increase in power (110W as opposed to the earlier versions which were 70W).

The earlier versions were also said to sound the best! My ST-140 is the second version (70 W).

If you know how to pop the hood, you can have a look inside and report what you see (If you're interested in knowing which version you have).


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[newindustar]

It is confusing. You saying A+B is 2 left speakers and two rights so still 2 LR channel right? On my old Sony STR-7055 (55 watts per channel) it actually as three speaker pairs A,B,C, twelve screw posts total. Nowhere is the word channel used. I can run A or B or C or A+B or A+B.

On my Pioneer 35 watt 6500II it has has A and B. I can run A or B at 4 ohms, A+B requires 8 ohms. I think the word channel should not be used with the A B C. perhaps just pairs A B or C. Each pair has two channel LR. Correct? I notice with the old Sony running two pair together does not increase sound level. However with a 135 watt Yamaha 450 receiver it does.

When running A+B or the amps cut in half, divided for each pair? What happens to the impedance? So it seem the word channel should be reserved for left or right.

I will open up the BK at some point. I know one version is called a toroidal version.