The Sound of New Transistors in Old Amplifiers

[All_DIY_STUFF]

Hi everyone,

I’d love to hear your opinions and insights about how new transistors affect the sound in vintage amplifiers. I know this is a complex topic, but after years of repairing and refurbishing old amps, and performing numerous tests, I’ve come to believe that transistors have their own sound signatures. This is especially true for transistors in the early stages of the VAS (Voltage Amplification Stage). However, there are times when I notice no difference at all. This inconsistency has been driving me a bit mad, so I wanted to share my observations and get your thoughts.

Here are a few examples from my experience:

1. Tone Control / Preamp Section:
   When I replace transistors in this part of the circuit, if I use KSC1845, the sound becomes overly bright, and the lower mids (especially vocals) seem to recede into the background. This is something I’ve noticed with most KSC-series transistors. On the other hand, when I use BC-series transistors, the sound is much more balanced, with everything sitting naturally in the soundstage.
2. VAS Stage (After the Differential Pair):
   Here’s where things get interesting. If I replace transistors in the first stage of the VAS, the effect is reversed: KSC transistors sound perfect, while BC-series transistors seem slightly off.
3. Differential Pair (Long-Tail Pair):
   When I create a matched pair of KSC transistors for the differential stage, I don’t notice any tonal difference. The sound does become cleaner and more detailed, but the tonal characteristics remain unchanged.
4. Preamp Circuit with Adjustable Voltage/Current:
   I built a preamp circuit where I could adjust the operating voltage and current to use the transistors in their most linear region. Even in this setup, the differences in sound signature persisted: KSC transistors sounded bright and less harmonic, while BC transistors were much more pleasant and musical.
5. Driver Transistors:
   The differences are even more noticeable when using these transistors as drivers for the final stage.
6. NOS NEC Transistors:
   I’ve also tested a batch of NOS (New Old Stock) NEC transistors. Most of them sounded terrible—cheap and uninspiring, with the exception of some power transistors, which were passable.
7. Output Transistors (Final Stage):
   For output transistors, I don’t notice as much of a tonal difference, but I do have a personal preference for Toshiba 2SC5200 and 2SA1943. These seem to deliver a more harmonic sound and often provide a better soundstage in many amplifiers.

My Questions for You:​

• Have you noticed similar sound differences between different transistor types?
• Do all transistors have specific sound characteristics? If so, what causes this?
• How do you choose the best transistor for a given application?
• In your experience, what are the best-sounding transistors for audio circuits?

I’d really appreciate your feedback and experiences. This topic has fascinated me for years, and I’m curious to know if others have noticed the same patterns or if my observations are more specific to certain circuit designs.

Looking forward to your thoughts!

 

[WhiteDragon]

Probably a certain design. Makes more sense with end of life transistor that could make such a difference.
When replaced the circuit works correctly. With difficult speaker loads at high volumes.
Only notable difference I heard. Just weak amplifiers. Average speaker in home settings. Never noticed much difference.
Big difference if you have nice tweeters. Is the hissing of some new amps.
Major difference was small amplifiers doing fine, watching a movie or in social gatherings. All of a sudden they shut down.
Modern amps of various sizes, just shut down. That " sounds" different.

 

[totally analogue]

@All_DIY_STUFF I tend to agree. I find the BC series more old school in the way they sound. Power transistors yes the Toshiba 1943/5200 and not forgetting the Sanken SAP15 (easy to destroy the internal bias resistor) and their great 2SA1216/SC2922. Sadly neither are now available.

 

[All_DIY_STUFF]

omg thank you, so im not crazy or at least not that crazy .

 

[njswede]

I’m pretty new to this whole audio thing, so bear with me. But how can you make blanket statements about BC transistors? Isn’t it just a numbering scheme? There are many different BC transistors from many different manufacturers. What do they have in common that changes the sound?

 

[analog_sa]

Unfortunately i don't have much to contribute here, but the topic is certainly interesting. Here is an older observation upon the sound of BCxxx transistors 🙂

https://www.diyaudio.com/community/threads/jean-hiraga-mc-circuit.4191/#post-38860

 

[All_DIY_STUFF]

This is a very good question .... I cant really answer that cause i only buy onsemi transistors so i cant give a difference between manufacturers. But all bc's sound the same and all ksc's sound more or less the same, usually bright .

 

[All_DIY_STUFF]

@analog_sa, in jean hiraga class A amplifiers the difference is night and day . On class A the sound difference is much more intense .

 

[Conrad Hoffman]

I'd never admit to hearing differences like that in the light of day, but I've got my suspicions. If you hear a difference, you can measure a difference. I've never found an exception that rule. The transistors vary in gain, sometimes by a lot. This can make a big difference is simple 1 or 2 transistor RIAA circuits, so I'm pretty careful there to get parts similar to the originals, plus I measure with an inverse RIAA network just to be sure. The VAS transistor provides all the voltage gain for most amps. It makes sense that the location would be a bit fussy in terms of linearity and operating point. Check for oscillation, THD and recovery from clipping. Don't know about drivers and outputs as our choices seem to get more and more limited there.

 

[totally analogue]

But how can you make blanket statements about BC transistors? Isn’t it just a numbering scheme?

Originally BC transistors were from the Philips Mullard Group and back in the day different manufactures had their own numbering system and ways of manufacture. Today I guess you are right, with most components being made overseas who knows what's under the hood?