Following is a testimonial from someone who had his Bryston 4B modded.
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I have owned my Bryston 4BST amplifier for 14 years and have generally been satisfied with it. The amplifier has been updated and improved three times since I bought it. I called Bryston, and they do not update this model. Due to the 20 year warranty and a solid design, you really don't see much online about modifying the Bryston.
I ran across a forum that discussed a modification done by Minsoo Kim, Master Technician at Pure Sound In Vancouver, to the differential input amplifier stage Bryston uses. It replaces resistors used in this stage with constant current sources. This was quite different from the common cap, resistor, HEXFRED, and wire mods I commonly see. Mr. Kim was adamant about the significant improvement it made to the amplifier. If someone did not like the results, he would change it back to the original condition at minimal cost, not leaving a trace of the modification.
I 'm not the kind of person who just takes someones word for it. I did quite a bit of research online and discovered that using constant current sources is the optimal way to run a differential amplifier. Minsoo Kim's claim was backed up by information on major university websites, as well as papers from major semiconductor companies.. It's solid engineering. An optimized differential amplifier reduces noise, second harmonic distortion and can improve dynamic range to a limited extent.
I called Minsoo Kim and asked him about the cost of the modification. It was very reasonable. Mr. Kim's customers seemed very pleased. Customers had high praise for his abilities. There were no posts about his modified amplifiers exploding or bursting into flames. I decide to ship my 4BST to Pure Sound and get the modification.
I have some words of warning to those who would consider this modification. I mentioned installing constant current sources to an engineer I spoke to. He said the person doing this has to really know what they are doing or very poor results may be obtained which can reduce the sound quality of the amplifier or worse. Mr. Kim is a Master technician. He can fix amplifiers without using a schematic. He has designed his own amplifier using constant current sources in the differential amplifier input stage. He has over 30 years experience with audio electronics. I will only endorse Minsoo Kim to do this modification. Hire someone else at your own risk.
I have over 150 hours of constant operation on my modified amplifier. There are no clicks, pops, buzzes, smells or anything abnormal I have observed in the Modified amplifier. It does look better as Mr. Kim cleaned it thoroughly.
The modification has only resulted in positives related to sound quality and are as follows
1. Listening Fatigue has all but eliminated. This is huge. I own modified Yamaha NS-1000Ms with the famous or infamous Beryllium dome drivers. The speaker has great clarity, but with the wrong associated equipment and crossover caps this speaker will drive you out of the room. I can listen for hours with no fatigue. No brightness. Amazing.
2. Far less grain. Smoother sounding. The grainy haze that permeated the sound stage is gone. There is a more "airy" presentation which adds much to the realism of the sound.
3. Less Congested sounding with less smearing. I am hearing details I have not heard before because it keeps all the different sounds separate. Much better real sounding decay, especially on piano. Everything is there. One instrument of higher volume does not cover up the other sounds of lower volume.
4. The general character of the amp remains unchanged, but there are differences. Piano sounds more solid with more low frequency impact. Plucked strings have more real texture. More solid and deeper low frequency drum sound. synthesizer are much more clearly presented, like hearing them for the first time. Ambient information is clearer, especially on something like Cowboy Junkies: The Trinity Session. There is an increase in what Srajen Ybaen calls color saturation, making voices and instruments sound more real.
5. Generally more clarity, however the clarity does not jump out and grab your attention. It is evenly and naturally presented throughout the frequency range. It is not "HiFi" sounding.
6. More subjective ease to the presentation. The amplifier never sounds strained or rough.
7. Mod provides outstanding value for the money.
8. I now just listen to, and enjoy the music. It just sounds good.
9. Comparisons. I've been out of the market way too long to say. I think it sounds better than the Threshold Stasis ClassA amps I sold during college. That's all I can say.
I think that Bryston is a great company. They have well deserved reputation for outstanding customer service, and equipment that is great sounding and extremely reliable. I have never regretted my purchase of the 4BST. My past interactions with James Tanner and Christopher Russell have been excellent. I asked Chris Russell one time about HEXFRED diodes in amplifier power supplies. He procured them, installed them in an amplifier, tested and listened to the amplifier, and then called me with a report. How many companies would do that for a customer? Zero, Nada, Nil, None. That's the kind of company Bryston is.
Minsoo Kim regards Bryston as an excellent amplifier design. He only disagrees on one thing, being the absence of constant current sources in the differential amplifier input stage. Mr. Kim has proved his point to me. The improvement is undeniable. With all due respect to James Tanner, Christopher Russell, Stuart Taylor, and rest of the great people at Bryston, I think that current sources in the differential input amplifier stage may deserve a second look.
