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Old 15th November 2002, 01:29 AM   #1
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Default Impedance matched amp

Regards DIY. So here's my first time posting story. My dear brother, a softy for a sob story, bought some gear sight unseen to aid the friend of a friend. He described it as some Bose things and an amp that did'nt work. Visiting him I found it to be a pair of 901s, (no comments here please, it's an amp thread), and a McIntosh C11. I suspected there had to be at least an impedance device or Bose amp but he showed me a vintage Sherwood receiver hooked up to, oh yes, an impedance/low filter box driving a series of in the ceiling speakers of his Mexico City rental. I informed him the bad news was the C11 was not an amp, but the good news was, well, you know. The other bad news is the little Sherwood receiver must have been used in the original system for it's tuner out function, leaving him still without a proper amp. I would like to build him an amp to go behind the C11. I'm thinking of either an Aleph 30 or an X thing with a Balanced Line stage as an interface to convert the c11 to balanced. I would probably fix the gain as low as possible to reduce distortion. Besides deciding which way to go, Does anyone have any thoughts on matching with the Bose's low impedance? In the interest of noise reduction, I'd like to forgo the impedance box. I think it also has some value driving his ceiling speaker array from a consumer receiver that won't tolerate heavy speaker loads.
Thanks for your thoughts.
p.s., my only experience so far is a balanced line stage I slipped into an Advent 300 I'm using as a tuner/pre-amp.
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Old 17th November 2002, 10:49 PM   #2
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Nobody seems to be replying here....

The Bose 901's like a lot of power, mostly because
(as I recall) they are heavily equalized. Does your
friend have the equalizer box? Is this the impedance
box to which you refer?
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Old 18th November 2002, 01:09 AM   #3
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Mr. Pass,
Thanks for the reply. I had only a brief visit with my brother, and I don't know if the interface box was an equalizer or not. It had two sets of controls: a low filter with settings something like 5, 10, 20hz and a control for impedance with settings like 2, 4, 8, 16 Ohms. I assumed the low filter was something like a rumble filter. The Bose's don't have any markings left and he did'nt have an VOM handy. Although they may take a lot of power to drive, I think he has settled into a more appropriate volume appreciation as he's aged. (I can say that only because I'm his older brother.)
I certainly appreciate your thoughts on an appropriate amp design.
Charles
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Old 18th November 2002, 01:12 AM   #4
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Default No Highs, No Lows, That's Bose (Without Eq)

I'm a bit confused by your post too.

Nelson is correct, Bose 901's had a small line level equaliser box to give nominally flat response from the cabinets.
Bose's low impedance? - standard 4 ohm cabinets from hazy memory.
And Nelson is right that they need horsepower to run them - a big old solid amp is needed to get them to perform (blow your curtains around).
Watchout (inspect) for dead foam roll surrounds too - there are 901 re-roll kits available.

Eric.
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Old 18th November 2002, 01:20 AM   #5
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Eric,
The box I found was certainly not small. It was about the size of the McIntosh C11 or bigger. I'm sure your advice is good regarding the foams, the power and the equalizer. Is it possible the C11 had enough tone control to do the equalizing job, or am I way off base?
Charles
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Old 18th November 2002, 01:23 AM   #6
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Eric and Nelson,
Sorry I did'nt mention it, the box I found with the impedance and low filter controls was definitely at the amp output.
Charles
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Old 18th November 2002, 09:24 AM   #7
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Quote:
Originally posted by Charles BL
Eric and Nelson,
Sorry I did'nt mention it, the box I found with the impedance and low filter controls was definitely at the amp output.
Charles
Does this box have a manufacturer name and model - would be a help.
Or a digital photo !.

"Is it possible the C11 had enough tone control to do the equalizing job, or am I way off base?
"

The Bose Eq boosts by 12 dB at 50 Hz and 14 dB at 10 kHz from memory.
With tone controls and loudness switch, you'll probably get near enough.

Eric.
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Old 18th November 2002, 11:46 AM   #8
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Eric,
I've written my brother for more info on the mystery box. I wonder how Bose managed that much boost without phase shift, or do you think they accepted the shift because there were'nt competing speakers? Anyway, I'll probably go ahead with something to drive his 901's for now, like an Aleph 30. I can talk him into some homebrew TL's or such when his stint in Mexico is over.
Thanks for the advice.
Charles BL
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Old 18th November 2002, 10:04 PM   #9
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If he doesn't have the equalizer, you'll have to build one.

(or, probably easier)

Toss the 901's out of respect for their age and start fresh.
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Old 19th November 2002, 02:22 AM   #10
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Nelson, I suspected it would finally come down to this. Well I'm not really hurt because I've always been fond of efficient systems and felt even the best of the low efficiency "cone" speakers sounded gaspy. Anyhow, everything has way of working out. Instead of info from my brother on the "box", he informed me he will be home by Christmas! I think the plan will be to E-Bay the 901's and use the take to buy some efficient drivers. And THEN on to the Aleph!
Thanks
ps, the other thing...I'm thinking of using a pearl like circuit for the latter stages of a tuner thats hurt...wonder if a ceramic filter will fit between the jfet stages...
ps, the other other thing...could a few RC shunts around the reference diodes in a power supply work as a soft start? Kinda work your way up the reference.
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