The AI-1 Dreadnought Project Pt.2 - Upgrade to 1000VA Transformer

I recently asked Ken Swauger if he had the original AI-1 specs. He did not. He referred my inquiry to Stu Lumsden (Polk's engineering VP) and Matthew Polk. Here is Matthew Polk's response:

"The original isolation transformer was successful at eliminating the issues of SDA with non-common ground amps but at the cost of a significant compromise of the SDA sound stage. We later figured out how to do this right with the transformer incorporated into the SRT controller.

The transformer is a 1:1 ratio and allows the L-R and R-L SDA signals to be derived while isolating the grounds of the two amp channels. The inverted portions of the SDA signals only need a bandwidth from a little below 100Hz to 1.6kHz. It's helpful if the transformer has more bandwidth on the top end since the high-end cutoff is controlled by a low-pass in the cross-over. At the low end, however, it's best if the transformer windings have a very high inductance as measured with the secondary open, around 14mh, and a very low DCR.

In the mid-range where SDA operates, the SDA drivers on one side operate in series with the SDA drivers on the other side with the signals passing through one set of SDA drivers then through the transformer and then through the set of SDA drivers on the other side. However, at low frequencies the positive part of the signal passes through the SDA drivers and is shunted directly to ground by an inductor in parallel with the transformer. The result is that the SDA drivers operate in parallel with the stereo drivers at low frequencies but operate in series with the other side to produce the SDA difference signal in the mid-range. If the DCR is too high, as it was in the original isolation transformer, it will reduce the level of the SDA signal and alter the bandwidth. If the inductance isn't high enough, also the case in the original, it will raise the low-end cutoff of the SDA signal further reducing the SDA soundstage and allow the SDA drivers to produce too much upper bass thereby muddying the sound.

The transformer in the SRT controller had a DCR around .3 ohms and an inductance around 14mh as best I recall. Current carrying capability before saturation is also an issue and was not adequate with the original. I think it was only about 3 amps which would be about 60 watts to the SDA drivers and not enough. I don't recall exactly what it was in the SRT but seem to recall something like 10 amps which would deliver about 600 watts to the SDA drivers.

As a practical matter we found the only way to get a low enough DCR was with a 1:1 toroidal transformer. These are available off the shelf but expensive. Because of the efficiency of the toroid the cores must be much larger to avoid saturation.

Hope that helps.

-msp

I measured the DCR and winding inductance of my original AI-1. Inductance was measured at 100 Hz with the other side shorted and again with the other side open. Measurements were taken inside the AI-1's case at the transformer terminals.

Original AI-1 Isolation Transformer Primary:
DCR: 0.22 ohm
Inductance (measured at 100 Hz): 13.88 mH (secondary shorted), 125.88 mH (secondary open)

Original AI-1 Isolation Transformer Secondary:
DCR: 0.25 ohm
Inductance (measured at 100 Hz): 14.25 mH (primary shorted), 125.95 mH (primary open)

=======================================================

Avel Lindberg Y236906 Isolation Transformer Primary (Dual Windings
Parallel Connected):
DCR: 0.28 ohm
Inductance (measured at 100 Hz): 6.01 mH (secondary shorted), 197.59 mH (secondary open)

Avel Lindberg Y236906 Isolation Transformer Secondary (Dual Windings Parallel Connected):
DCR: 0.38 ohm
Inductance (measured at 100 Hz): 6.55 mH (primary shorted), 215.44 mH (primary open)

=======================================================
Avel Lindberg Y236906 Manufacturer Supplied Specs:

Avel Lindberg Y236906 Isolation Transformer Primary (Dual Windings Parallel Connected):
DCR: 0.275 ohm
Inductance: 153.3 mH

Avel Lindberg Y236906 Isolation Transformer Secondary (Dual Windings Parallel Connected):
DCR: 0.360 ohm
Inductance:168.8 mH

======================================================

Avel Lindberg AV-047548 Isolation Transformer #2 Primary (Dual Windings Parallel Connected):
DCR: 0.24 ohm
Inductance (measured at 100 Hz): 3.75 mH (secondary shorted), 262.55 mH (secondary open)

Avel Lindberg AV-047548 Isolation Transformer #2 Secondary (Dual Windings Parallel Connected):
DCR: 0.29 ohm
Inductance (measured at 100 Hz): 4.10 mH (primary shorted), 286.8 mH (primary open)

======================================================

In the case of the original AI-1 and A-L Y236906, there were some differences between the manufacturer's specifications and my transformer inductance measurements. Inductance varies with frequency, and I do not know the frequency at which the manufacturer's inductance measurements were taken. MY MCP BR2822 LCR meter generates test frequencies at 100 Hz, 120 Hz, 1,000 Hz and 10,000 Hz. I could not match the manufacturer's inductance specs at any of those frequencies.

My Y236906 DCR measurements agreed with Avel Lindberg's specs. My primary and secondary inductance measurements were approximately 45 mH higher. The ratio of primary to secondary inductance of my measurements was close to Avel Lindberg's"

My Y236906 open circuit inductance measurements: 197.59 mH (primary)/215.44 mH (secondary) = 0.9171

A-L's Y236906 open circuit inductance measurements: 153.30 mH (primary)/168.80 mH (secondary) = 0.9082

I wonder if there were different versions of the AI-1. Matthew Polk said the AI-1 had a very high transformer winding DCR, but I measured only 0.22 ohm and 0.25 ohm for the primary and secondary. I did measure close to the specified 14 mH primary and secondary inductance, but only with the other side shorted. In the open circuit case, the inductance of both sides was approximately 126 mH. If the open circuit inductance is the required parameter, then each of the A-L inductors

Original AI-1 primary and secondary open circuit inductance (at 100 Hz): 125.88 mH and 125.95 mH.

Avel Lindberg Y236906 primary and secondary open circuit inductance (at 100 Hz): 197.59 mH and 215.44 mH.

Avel Lindberg AV-047548 primary and secondary open circuit inductance (at 100 Hz): 262.55 mH and 286.8 mH.

The Jantzen 0.65 mH/26 AWG/ inductors were not appropriate substitutes for the 0.7 mH/25 AWG original inductors. There were detrimental bass effects: loss of weight, detail, and articulation. Tactile sensation was also diminished. Vocals and solo instruments in the center also appeared to move backward a couple of feet. I thought (hoped) these would work since the Jantzen part is within 7% of the original part's nominal value and the Jantzen part's DCR is within 4% of the original part. The original 0.7 mH inductors were reinstalled.

The Jantzen 0.4 mH/22 AWG inductors provided a little more detail, particularly with vocals, than the original 0.4 mH/22 AWG inductors.

Details coming later in a separate thread.

DarqueKnight wrote: »

The Jantzen 0.65 mH/26 AWG/ inductors were not appropriate substitutes for the 0.7 mH/25 AWG original inductors. The original 0.7 mH inductors were reinstalled.

Toolfan66 wrote: »

I have mine but it will be a couple weeks before I have time to install them...

I am going to order some 0.75 mH Jantzen inductors (26 AWG, 1.80 ohm DCR) and unwind them to 0.7 mH. My report will be submitted after I have tried the unwound 0.75 mH inductors.

The Jantzen perfect lay inductors appear to be comparable in performance and construction to the Solen perfect lay inductors. The Jantzen inductors sounded harsh in the highs and smeared in the bass upon first listening. The sound significantly improved after a few hours of play time. I did not condition the Jantzen inductors on the Cable Cooker as I did with the Solens. I didn't feel like going through the numerous removal and install iterations.

In contrast to the Jantzens, the Solens sounded better than the original inductors right from the beginning and got better with time. I replaced all of the on-board inductors (0.4 mH, 1.25 mH and 2.5 mH) in my three pairs of CRS+'s with equivalent Jantzen parts and I replaced the two 0.4 mH inductors in the high frequency section of my 1.2TL's with Jantzen parts.

thsmith wrote: »

Plitron shipped today, tracking does not show a delivery date yet but will post when it is available.

OK. Thanks.

Toolfan66 wrote: »

What about winding more to the 0.65 to get the 0.7 ??

That's not something I'd want to get into. I would have to acquire the exact same wire, solder it to the existing leads, neatly wind it, then glue it in place.