Bi-Amping Von Schweikert speakers

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duguyisheng

Re: Bi-Amping Von Schweikert speakers
« Reply #80 on: 2 Jan 2017, 02:22 pm »
I have been wanting to experiment with biamping but without a big expenditure and without worrying to much about sensitivities. After reading this thread, I found an older NAD amp as suggested on the board and it has a dial to adjust output for biamping. I then played some pink noise and using a multimeter adjusted the output so the readings on the upper and lower modules of my vr5 were about equal. it took just a few minutes to do this. For the top, I was using a BAT tube amp.

I am a believer. I knew the top amp was never strong enough to give me good bass. Now, I get good bass and the upper end sounds less congested (actually not congested) with big orchestras. Big difference. I am now ready to experiment with other big amps for the bottom. This has been a huge difference in my system.

kernelbob

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Re: Bi-Amping Von Schweikert speakers
« Reply #81 on: 2 Jan 2017, 11:29 pm »
I've been biamping VSA speakers for years, across three different models.  Currently, I'm using Tortuga LDR attenuators for both the primary preamp/controller that switches between inputs and another one with only one input to adjust the level of the bass amps.  I would recommend using a primary controller and bass amp attenuator that can be adjusted from the listening position.  Once you have the capability to fine tune the level of the bass amp relative to the mid/treble amp, you really come to appreciate the control.

I've found that different recording labels have very different approaches to the bass/mid-treble balance.  For example, the bass on Naxos recordings is unfortunately not as extended or dynamic as it should be.  In a word, it sounds polite.  I frequently bump the bass one or two dB's compared to, say, a London/Decca.  Similarly, different streaming services each have their own frequency profiles and often need a bass amp adjustment up or down a dB or so.

One other recommendation that I'd make is that the primary preamp/controller should include the capability to toggle the absolute phase of the system... from the remote.  Once you use that for a while, you won't want to be without it.  FYI, my Tortuga LDRxB has that feature.  The difference between the right and wrong setting is clearly audible on a reasonably good system.  Unfortunately, there's no consistency between labels (and even recordings within one label), so you'll need to confirm that you have the right setting for each recording.

Best,
Robert

mhconley

Re: Bi-Amping Von Schweikert speakers
« Reply #82 on: 19 Jun 2018, 11:49 pm »
From Albert Von Schweikert:
3.  The "secret" to achieve "killer" sound quality is to ensure that the amplifiers have exactly the same input sensitivity.  The amplifier with the lowest numerical rating, i.e. 100mV is much more sensitive than an amplifier with a rating of 500mV - the higher the number, the lower the sensitivity.  You'll need to reduce the higher sensitivity by using a series input resistor.  This can be installed inside your amplifier, directly at the RCA female jack leading to the input stage. If this sounds a little scary, then build an adaptor to house the resistor outside the amp.  If you contact Michael Percy at www.percyaudio.com, he will advise you on what you'll need to get from him.  Usually, a female RCA jack, a male RCA plug, a high quality metal film resistor, some plastic sleeving, and a short piece of high quality hookup wire is all that is required.  Basically, you're inserting the resistor between the "hot" connection from the female RCA jack to the male RCA plug, and then using hookup wire to connect the ground leads.  The entire assembly can be only a few inches long and installed into a plastic or Teflon sleeve to prevent short circuits.  This adaptor is inserted into the signal path between your preamp and the most sensitive amplifier. As you can infer from this description, the adaptor is used in between your interconnect and the input jack of the amplifier.
Michael Percy sells these parts for less than $100 for everything you'll need, and he'll also help you select the proper value of resistor to match the sensitivities.  Although this simple technique may sound like a "Micky Mouse" setup to "sophisticated" engineers that design chip OP AMPS and sell electronic crossovers to the PA industry, it is a very "pure" form of passive matching that will stomp the crap out of any electronic crossover I have ever tried.

Assuming I know the input sensitivity and gain of each amp - what is the math to calculate the resistor value?

Specifically, I am looking to use my DIY ICEpower 1000asp based monoblocks on the LF cabinet (2.96V input sensitivity) with a pair of VTA m-125 monoblocks on the HF cabinet (1V input sensitivity). From Albert's statement above the m-125 is more sensitive than the 1000asp so I need to add a series resistor to its inputs. Since I am building the m-125 from a kit I want to add the resistor inside the amplifier. What value will I need?

Martin
« Last Edit: 20 Jun 2018, 01:25 pm by mhconley »