Threshold CAS-2, Blast from the past

I recently had the good fortune to find a used Threshold CAS-2 at one of my local high-end audio stores. It has been a pleasure to listen to and learn about the history of one of the earliest Threshold designs from the late 70s.

The unit was well cared for by its previous owner(s). If anything, it may have been in clean storage for a few years. Initial power up resulted in a significant amount of mechanical 120Hz hum from the internal power transformers. This subsided a little over the next couple hours as I let the amp come up to temperature. There was also a small amount of 120Hz hum present from the speakers, but this component was not noticeable at my listening position. Initial listening was good, with a noticeable amount of brightness to the tonal balance. The brightness mellowed out slightly over the next few hours of powered operation.
I switched to a Transparent power cable for the next day's listening. This power cable is an earlier 1.5 meter model that has three lumps of filter network along its length. I have used it in the past with amplifiers that had sensitive power transformers. The Transparent cable did its job once again, and both the mechanical hum and audio hum were noticeable reduced. Listening to the amp with this configuration proved to be a lot of fun. In many ways the CAS-2 reminds me of my old olive era Naim NAP250, in the best of ways. The CAS-2 is lively and engaging, with a distinct sense of Pace, Rhythm and Timing, as Naim aficionados like to say.
Over the next two days of listening sessions I found the amp to sound better integrated over the whole frequency spectrum. The imaging solidified as well, and there started to be a better sence of the distinct harmonics of acoustic and simple electric instruments. The bass developed good extension and definition, sometimes calling more attention to itself than I am used to on a few recordings, though this feature also mellowed slightly over time. The amp definitely sounds its best when it is left on for a few hours, so I started leaving it on all the time. The case gets just slightly warm after a full day. This is not a Class A design, but a moderate high bias class A/B. It appears to be an excellent match with my newer Vandersteen 3A Signatures, with plenty of power available for good control. The dynamic presentation seems to be better with the CAS-2 than with my DIY M2x. I have enjoyed revisiting a number of my favorite recordings to hear them with a better set of dynamics.
The CAS-2 also compares very favorably with my DIY F6. The F6 has been something of a test bed lately, as I was experimenting with some alternate Mosfets in the output stage. The sonic comparison with the CAS-2 led me to re-think some aspects of my F6 modifications, and I intend to try some new ideas to help the F6 provide some of the same rhythmic enjoyment that the older amp seems to provide with ease.
 

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Yes, lots to look forward to with this one. 🙂

There is an older thread (now closed) that had some good info on the older amps:
https://www.diyaudio.com/community/threads/general-question-old-threshold-equipment.30258/
In short, it is probably time to do some careful re-capping of my 30 year old CAS-2.

I have experienced the effects of a set of power supply capacitors aging past their useful life, in that old Naim NAP250 in fact. Nothing dramatic, but there was a occasional sound of static coming from the speakers and other sonic oddities. Naim recommends re-capping their amps and preamps about every 20 years, so this is not unusual. I think some of the things I have experienced with my CAS-2 are due to the big supply caps essentially re-forming themselves over the course of a couple days. There is actually a specific capacitor re-forming procedure that may be performed on caps that have remained in storage for a few years. I wasn't nearly that fastidious, as I was already planning to replace the big electrolytics, plus others that might be aging out.

Examining the inside of the CAS-2 was a treat. It is definitely a dual-mono amp, with separate power supplies that share only the inlet line fuse and power switch. The grounds follow a star wiring topology. Each channel boards has at least three separate ground wires that tie into the center of the aluminum ground plate between the bulk filter capacitors. The speaker ground connections are also brought back to the central ground point. Speaking of the big bulk capacitors, notice that there is a single one for each power rail. Not a CRC filter or anything that fancy, just a single large cap (Mallory 6900 uF, 60V) that was chosen to be compact and have long life. Overall wiring is done very neatly, with similar attention to detail as the old Naim 250. It is gratifying to see this in an old design. It was probably one of the first commercial US-made amplifiers to be constructed with such care. The star ground implementation is probably one of the more important aspects that give this amp the excellent staging it has.

Discussion in the Pass Labs forums has only yielded some other photos and a set of schematics from the CAS-1. I can say from examination that the CAS-2 differs significantly enough from the CAS-1 that those schematics are only useful in the most general terms. The CAS-2 is completely DC coupled from input to output and has more than double the transistors, both large and small cases. While the bulk power capacitors appear to be the same, I haven't measured the supply voltages yet. Having too much fun just listening so far.
 

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These old Threshold BJT amps have a neat sound.

I bet you can squeeze a nicer power supply in there. Maybe a CRC board with smaller snap-in caps in multiples stacked with tall standoffs. Well, I guess that depends on the voltage. Adding a snubber circuit for the transformer secondaries would be an easy trick.

If I am understanding you right, the signal ground is tied directly to the chassis ground. are you going to try lifting it with a CL-60 or something?

I had some Vandersteen 2ce's for a little while. They were great but needed a lot of muscle to get them moving. Probably one of the most stubborn speakers that I have driven.
 
I began a limited amount of re-capping on the channel boards two days ago, with very positive results.
While I was prepared to replace an input coupling capacitor, there simply isn't one. Easy.
The next capacitor due for an upgrade was the feedback coupling cap. In the CAS-2 this is a 470 uF, 16V bi-polar component. I had a few of these available in the Nichicon ES series (green sleeve), so I replaced the originals with these and stopped there to listen.

Substituting the Nichicon ES series cap in the feedback path gave a remarkable improvement. I gave the amp about 2 hours to warm up before listening. There was an immediate sense of better high to low frequency integration and improved spatial dynamics of instruments. Then after a couple more hours of listening (the CAS-2 is addictive that way), my big Vandersteens simply disappeared. As in there was no physical tie of the soundstage presentation to these big 50 inch tall black monoliths in my living room. Instruments and ambient sounds from the recording space extend above and beyond the speakers from side to side, and now go deeper from front to back. The instruments, particularly ones with percussion aspects have a well defined physical location. The Vandersteen speakers are designed to perform at the highest level in this area, and the CAS-2 started to reveal things in some of my favorite recordings that I hadn't heard before.

Thank you Papa Nelson for a gift that keeps on giving after so many years.
 
The plan from this point is to make a few more careful updates of the older caps that are close to aging beyond their best performance. Given the sonic performance of the amp in its current state, I am not inclined to make large changes that may alter the character.

For example, I haven’t figured out what technique is used to separate audio ground from chassis ground, though they certainly do appear to be separate. There are a couple higher value resistors (~ 3k) between the RCA input shields and the chassis. While the overall scheme would be interesting to know, I would not alter it.

I am currently waiting for a set of Kemet ALS30 series 15,000 uF, 63V capacitors to be delivered. These are high CV efficient, very long life components which should be good replacements for the Mallory parts that are already in use. This will be a one-for-one substitution, with full care taken to avoid any issues from their extra height. Following this will be a couple more days of listening before I consider anything else.
 
Time for an update on the work with this lovely amp.
I received the new Kemet caps to replace the original Mallorys. They were a good fit once I fabricated a new ground plate. The extra capacitance resulted in both measurable and sonic improvements. The DC rail voltage reads 300 mV higher on my meter than it did before. Hum at the speakers is only detectable with my ear next to the midrange driver. Very worthwhile.

The next thing I wanted to try was replacing the the 47 uF caps that set the cascode voltage. I had Mundorf bipolar 56 uF RAW dielectric caps that fit well onto the boards.
The re-capped amp has had over 200 hours to settle in with the new parts and sounds amazing. It is dynamic, lively, and smooth. The soundstage stretches above and beyond my speakers with fine instrumental location and detail. I’ve been enjoying a variety of recordings over the last couple weeks to hear what they sound like with this amp. Lots of fun.

The CAS 2 has inspired me to return to the New Stasis amp project with ZM’s input stage and 8-deep OS boards. I need to hear what this design sounds like with modern parts and non-cascode output stage.
 

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