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Old 8th March 2011, 06:54 PM   #81
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Quote:
Originally Posted by gnnett View Post
Hi Andy

Good to see you here. I posted the following question on Audiocircuit but if you don't mind will repeat it here.

This is in regards to the wiring of the transformers for the Spectra 44.

I am wondering if the pair of transformer primaries can be wired in series, rather than parallel to achieve an easier load for an OTL amplifier.

I have sucessfully used an autoformer (the zero) with my 1+1 and an aquaintence had great sucess with Model 6's, by wiring the two MK121 interfaces in series, with a similar OTL to mine.

I am guessing that if it is possible there might need to be a change to the resistor and capacitor network on the primary side of the transformers.

I realise that the results will be unknown until I try it, but checking to make sure there is no obvious fatal flaw with the idea.

Kind Regards

Grantn
I don't see any major problems in running the two transformers in series, except perhaps its effect on damping factor. That is, each transformer is normally directly connected to the amplifier, but in series mode, you have another transformer in series which will effectively halve the damping factor. But, since you are using an OTL amplifier, which typically have a high output impedance, your damping factor is zilch already, so perhaps this is not an issue for you. Me, I'm a card-carrying member of the High Damping Factor Club.

I would not expect any changes to be required of the RC input network. The 1-ohm resistor in series is there to prevent the impedance from going below 1-ohm at very low frequencies. The capacitors in parallel with the resistor bypass the resistor at higher frequencies.
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Old 8th March 2011, 07:07 PM   #82
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Originally Posted by charlesp210 View Post
I've just been thinking about the 3 capacitors in the "C" version Medallion interface as compared with previous interfaces. The total capacitance in the "C" version is 57.01uF (47+10+.01). The previous versions used a 220uF capacitor, or 220+10+0.1 for a total of 230.01uF in the "B" version.

I'm been unable to find actual schematic of "C" version (my owners manual shows the pre-medallion version even) and I'm wondering if these changes were made without changing the network somehow to achieve the same relative high-pass effect.

I have heard the low frequency transformer runs as full range as it can in both versions. So unless other changes were made, simply reducing the capacitance from 220 or 230.01 to 57.01 is going to have some effect on the overall frequency response because the HF transformer will be contributing less at it's lower end. A loss of output in the midrange, for example, around 2khz might be expected. So given the ability to do so (which I'm thinking about adding, during my current still-in-progress capacitor modification) changing the capacitor to suit room, taste, etc., might be desireable.

On the other hand, if you have the pre-C version, you might get more high frequency headroom (and less HF transformer melting) by changing to the "C" version capacitor values. I also thought I had heard or read here that one of the motivations for the "C" modification was to increase high frequency headroom.

Now I'm wondering if that didn't have some affect on the frequency response also, and perhaps one that might not be preferable to all listeners.
The Acoustat C-Mod was NOT just a change in capacitor value. The high-pass filter feeding the HF transformer changes from a series network to a series/parallel network, resulting in a smoother roll-in and less chance of core saturation. Merely changing the capacitor value will NOT give you the C-Mod, and doing so is not recommended. The resistor values and network configuration must also be changed.

While it is true that the LF transformer does not have a low-pass filter associated with it, it does not really operate full-range due to its natural roll-off.

Schematics and instructions for the C-Mod are available from www.audiocircuit.com. Note that doing the modification is beneficial for both original and Medallion transformers: they are independent changes. The parts required are commonly available, and the instructions provide several options for creating the new HF Balance Control. The 16-ohm slider resistor required for interfaces so equipped may still be available from eBay seller Soundvalves, who bought most of the obsolete Acoustat parts inventory from Rockford. If not still available, the instructions provide a way around that.

One nice thing about the C-Mod is that the 47-uF capacitor is much easier to upgrade to a film-type, as compared to the original 220-uF (huge AND expensive!)

If you can't find the above referenced documents, I can post them directly here.
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Old 8th March 2011, 07:21 PM   #83
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Quote:
Originally Posted by charlesp210 View Post
The picture shows the inside of my Acoustat Red Medallion C interface with the new Solen 47uF capacitor in front. It looks like the new capacitor (bottom) will fit either in front of the circuit board on right side, or above big transformer in back. There's more space in back, but nothing convenient to support the capacitor with and a long lead would be required, and there's high voltage back there. I'm leaning toward front installation.

In front, the capacitor could rise straight from where the fuseholder is, or lie flat above the fuse and two speaker terminals. Either way space is very tight, and one concern is that the capacitor would be very close to the 50K power resistor. If the resistor gets hot it could damage the capacitor, and there is high voltage on that resistor too. I might be able to give it "just enough" space, it looks like I could give it about 5mm space. Opportunities for support are also limited, but it would be somewhat self-supporting from it's own lead, so doesn't need that much additional support. If it falls "down" in operation it simply hits the wood base of the speaker. where the interace is installed.

I could possibly bend the power resistor out of the way, bit by bit using one or two sets of longnose pliers. The danger is that I could break it off it's solder pads, or, even worse, crack the old circuit board.

Another thought crosses my mind too. I could connect the capacitor BEFORE the fuse, so the capacitor effectively bypasses the fuse for the high frequencies. The speaker would still be protected from DC and low frequencies, which is likely where any problem would be. The fuse doesn't really do a great job of protection anyway from sustained loud music, sustained loud music can burn the transformer without melting the fuse. The fuse mainly protects against amplifier failure, and also provides nuisance warnings that you are playing too loud.

I could also have the capacitor outside, either as an independent entity (with new input terminal) allowing easy changes, but clumsy and space consuming, or strapped to the case with twist tie (requires new holes in case, not easy) or glued (ugly).
I might suggest two options, either or both of which may ease the difficulty of mounting the 47-uF film capacitor.

The first would be to find a capacitor with a lower voltage rating, which would likely be smaller in size. You could also use two smaller values in parallel, which might open-up further mounting possibilities.

The second would be to use self-adhesive pads designed for securing cable-ties. Find a spot on the rear surface of the chassis, mount the tab, run the cable-tie through it, and secure the capacitor to that. Using the self-adhesive pad avoids any need for drilling holes in the chassis. I have used a similar technique in my Spectra's (which admittedly have a little more room available).
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Old 8th March 2011, 08:44 PM   #84
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Quote:
Originally Posted by AcoustatAnswerMan View Post
The second would be to use self-adhesive pads designed for securing cable-ties. Find a spot on the rear surface of the chassis, mount the tab, run the cable-tie through it, and secure the capacitor to that. Using the self-adhesive pad avoids any need for drilling holes in the chassis. I have used a similar technique in my Spectra's (which admittedly have a little more room available).
That is a great idea Andy about the cable tie mounts. There are mounts like that that also only need one screw if adhesive was not enough.
Here is a photo of my interface that someone else had done.121C.JPG

Last edited by speedracer5; 8th March 2011 at 08:48 PM.
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Old 8th March 2011, 08:50 PM   #85
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When I made my Spandex socks I ended up with the fabric being half the circumference of the speaker, so it ended up being stretched to twice its size. This yields a semi-transparent look where you can just make out the grids of the panels if you look closely. Under normal viewing they just look like blue monoliths. They are far more acoustically open than the stock socks, though; the highs are much clearer this way. I'd never consider going back to the old socks, both in looks and sound.
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Old 9th March 2011, 02:15 AM   #86
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Default Can't find actual C-Mod on The Audio Circuit

Quote:
Originally Posted by AcoustatAnswerMan View Post
The Acoustat C-Mod was NOT just a change in capacitor value. The high-pass filter feeding the HF transformer changes from a series network to a series/parallel network, resulting in a smoother roll-in and less chance of core saturation. Merely changing the capacitor value will NOT give you the C-Mod, and doing so is not recommended. The resistor values and network configuration must also be changed.

While it is true that the LF transformer does not have a low-pass filter associated with it, it does not really operate full-range due to its natural roll-off.

Schematics and instructions for the C-Mod are available from www.audiocircuit.com. Note that doing the modification is beneficial for both original and Medallion transformers: they are independent changes. The parts required are commonly available, and the instructions provide several options for creating the new HF Balance Control. The 16-ohm slider resistor required for interfaces so equipped may still be available from eBay seller Soundvalves, who bought most of the obsolete Acoustat parts inventory from Rockford. If not still available, the instructions provide a way around that.
I am almost never able to find *anything* on AudioCircuit because there are no cross-links. If I pull up the Acoustat page, there is no "modification" tab on the right (which seems implied by a number of posts, I know TAC itself has gone through many changes...not long ago it wouldn't even work on Macs). If I search for "C-Mod" I never find the actual C-Mod, I simply find endless comments like this:

"The C modification is easy to do... Most of the information is posted on this site... [Useless or outdated instructions for finding C-Mod information under "Modifications" (and where is "Modifications", you can't navigate to that from forum, etc.)]

**** Update. I think I found it here:

AudioCircuit http://www.audiocircuit.com/A-PDF/AA...N2-941-ACO.pdf

Last edited by charlesp210; 9th March 2011 at 02:30 AM.
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Old 9th March 2011, 02:51 AM   #87
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Default Navigating The Audio Circuit

don't start with SEARCH. Instead, starting at TAC home page, click on "Home Audio Section" which shows list of manufacturers, then click on Acoustat, then scroll down and down on page to "Refurbishing and Modifications", then subheading Andy Szabo's Technical Bulletins, then subsubheading modifications, and the last one is MK-121-2(A) Medallion "C" Modification.
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Old 9th March 2011, 03:21 AM   #88
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Quote:
Originally Posted by AcoustatAnswerMan View Post
The Acoustat C-Mod was NOT just a change in capacitor value. The high-pass filter feeding the HF transformer changes from a series network to a series/parallel network, resulting in a smoother roll-in and less chance of core saturation. Merely changing the capacitor value will NOT give you the C-Mod, and doing so is not recommended. The resistor values and network configuration must also be changed.
For those who read schematics better than they read words, here is another set of great circuit diagrams from Andy depicting what he just posted in words.
Attached Images
File Type: gif MK-121C_Xovr_changes.gif (26.2 KB, 396 views)
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Old 9th March 2011, 07:26 PM   #89
palmito is offline palmito  United States
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Default Pass F-3,4,5 or Burning Amp good for driving Acoustats 2+2

Hi. I own a pair of 2+2's and was wondering if anybody has used the Pass F-3 or F-4 or F-5 amps (bridged) with them? I'm looking to build an amp for them the Impasse preamp/F-x amps are highly rated. Thanks.
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Old 9th March 2011, 09:34 PM   #90
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Default Low Power Amps on Acoustat

I have not tried First Watt amps. I notice that they are spec'd at 25W at 8 ohms and 40W at 4 ohms. That doesn't sound adequate except for chamber music, etc.

When I first got 1+1's, I borrowed a Parasound HCA-1000A from my bedroom system. That amp got incredibly hot and even shut down while playing orchestral music. That's 125W/8 ohms and 200W/4 ohms.

The Acoustat amp has 200W/8 ohms and 300W/4 ohms. That's a sort of benchmark for what the speakers require, of course, though it's also said that beyond mere power ratings, it's designed to be well suited to the electrostatic load (which tends to reflect energy back into the amp).

I've had no trouble with Parasound HCA-1500A which has 205W/8 and 300/4, similar ratings to Acoustat amp.

There are a number of Pass Labs amps which look suitable...
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