To Biamp or not to Biamp, that is the question???????

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For what it's worth, I started an active crossover thread (yes, using JFETs...2N5457s, but no reason you couldn't use something else, including bipolars or MOSFETs) in the Everything Else forum a while back. There's more to do, but there's enough there to start someone who's thinking in that direction.
I'll get more posted in there one of these years...

Grey
 
Rodd.
Can you tell us which issue of Audio Xpress had the article on the Rane AC22?
This is exactly what I have and I would be very interested in more information on improving it's performance.

For general info to those who might persue this x-over, the AC22
is a stereo, 2 way unit and comes with the delay feature wired for the LOW channels but it can be converted to the HIGH channels. I suspect most of us on this thread would be using it that way instead of how it comes from the factory.
There are clear instructions in the manual on how to do the delay conversion.

Thanks, Timo
 
Re: To Biamp or not, that is the question???

macka said:

I would like to see a decent active crossover (ie J fet design) but until then I 'm sticking with the passive.


If you are looking for a good active crossover why not try this one: http://www.diyaudio.com/forums/showthread.php?s=&threadid=4098&pagenumber=2
It comes from Nelson Pass and, if not considering elaborate adjustments and variety of presettings, the whole idea is pretty close to his current model. You could prabably still improve it by changing semiconductors for better ones. I can only assume that it beats in all aspects.
 
The one and only
Joined 2001
Paid Member
The nice thing about the PCX was that all the resistors
were the same value, so you could pop in those 8
resistor chips (Digikey) into a socket and get a clean
18 dB/oct filter.

It got great reviews, particularly from Dick Olsher in
Stereophile.

The gain stages worked fine, as they only had a 6 dB
gain figure, and you can easily sub them with (ugh)
op amps.
 
Ex-Moderator
Joined 2002
Hi Timo,

I had to go to the non-active reading section of my porcelain library, but I found it. It is the December 2001 edition.

Mr. Pass, Bill,

I remember a preamp from the 70's, the Apt preamp by Thomas Holman. It was op-amps through-and-through. I know the switches and pots could have been better quality, but the sound was one of the most neutral sounding preamps around at the time. The price was right as well.

Just a thought.

Rodd Yamashita
 
The one and only
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Paid Member
I seem to recall that the Apt was a discreet, if not
particularly remarkable, circuit. Probably Holman's
best work.

I don't want to get off on a rant here but
I'll tell you what I don't like about op amps:

1) Huge open loop gain figures that I don't need at all

2) Marginal stability at low gain figures

3) Not designed for audio, no matter what they say

4) This is DIY. I view op amps the same way I look
at a can of soup or a fast-food burger

5) You can actually build your own op amps pretty
easily, and often more cheaply




:p
 
I have the APT owner's manual - complete with detailed schematic (nobody does THAT anymore!). Alas, it's at home so I'll post more tonight.

It used op amps (TL07x) which were good for their day. The RIAA stage was discrete. Also used a emitter follower with current sink load as a buffer for the volume control. The volume control was a feedback design that had low input impedance. TL07x are notorious for not liking to drive even moderately low impedance loads.

I actually duplicated some of the circuitry for my earlier preamps. Worked quite well and sounded very nice, especially with better performing parts.

Some of the audio magazines and electronics trade journals of the day had some great articles about the design Holman gear. Some things were very usual - I can't recall seeing any other amp that used the Baker Clamps approach to keep the voltage gain transistors from saturating (to keep the rails from sticking - a big problem with some of the "high slew rate" designs from that time period).

roddyama said:
Hi Timo,

I had to go to the non-active reading section of my porcelain library, but I found it. It is the December 2001 edition.

Mr. Pass, Bill,

I remember a preamp from the 70's, the Apt preamp by Thomas Holman. It was op-amps through-and-through. I know the switches and pots could have been better quality, but the sound was one of the most neutral sounding preamps around at the time. The price was right as well.

Just a thought.

Rodd Yamashita
 
The one and only
Joined 2001
Paid Member
Apt was around for quite a while after Holman split, and
in fact one of my ex sales manglers went to work for them.

I can't help but think the TL074's were a later innovation.

Also, I meant the circuit was discrete, and who knows if
it was discreet. :)

BTW, I always rather liked the TL074 and its brothers.
For op amps they were remarkably simple and had nice
FET inputs driving a single ended bipolar and complementary
followers. And they sounded pretty decent.
 
Ex-Moderator
Joined 2002
I had to have a relay replaced in my Apt preamp in '87 and I know that Holman went to Advent sometime in the early 80's; so there was a time that Apt Corp. continued on without Holman.

The op-amp he used most was the TL072CP. "Through-and-through" might have been a bit strong. This op-amp is used on all the inputs except in the phono circuit. It is used as a buffer in most of the control circuits and output circuits.

The point being that even though a lot of people don't care for op-amps, it appears that, done right, they can sound very good.

Rodd Yamashita
 
Bill Fitzpatrick said:
A lot of people don't seem to like op-amps.

I was wondering if anyone would object to an op-amp in the low pass section and discretes in the high pass.

Bill,

Thats basically what I have designed for my system. It's 4 way, triamped crossed at 120, 500, 6500(passive) with tube amps for the top two and SS on the bottom. I chose opamps because I found they're hard to detect there and because I need to EQ the bottom of the horns flat. Easier and cheaper with ops. If I'm bored next year I might re-do it all discrete. I have tons of gain spare so it might be easiest to do a discrete buffer followed by a passive shaping network before the bass PA.

The 120 HPF and 500 are passive before the poweramps; 6dB is enough here as the horns start to roll off out of band quickly anyway.

Cheers
 
To Biamp or not, that is the question???

On the subject of opamps,

A while ago in Audio Electronics there was a good series of articles on building a mixer using either various chips or discrete buffers/gain stages using a Borbley fet/bjt circuit. (1995)

The article refers to various performance aspects of chips and sights noise and offset voltage/current important in some applications pending what you are doing.

There is also the issue of drive current for chips and it is possible to extend class A operation by biasing the output stage with a simple current source or transistor.

The chips referred to were the TL072, Ne5532, OPA 604
From what I recall the fet/jfet types has low offsets (which maybe important in crossovers) but were some what noisier than the bi polar counterparts and the discrete opamps had the best of both worlds and sounded great.(Borbley)

The article also has excellent coverage of balanced operation, power supplies design and component selection.

If you are interested I could dig it out and arrange copies!

BTW Borbley has a couple of nice downloads in Jfet applications if you are interested in discrete opamps and has a crossover board and article for a preamp/crossover if you are interested.

best regards

macka





;)
 
Nelson, a quick question here.

At passdiy site under coming attractions is a project
"The High/Low Pass, the first word in electronic crossovers"

Can you please give us some peeks into this project :)

Especially
*When will you estimate (very roughly) it to be available?
*Is there going to be some sort of kit available (pcb etc)?

*******

I'm also planning to try biamping in the future. Your DIY project could form the basis...

Regards
Ergo
 
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