[WireNut]

I just purchased my first Class A designed power amp, It’s a Threshold T50 Class A amp with 50 watts per channel to drive my top MTM cabinets in a bi-amp system.
The T50 I have read is one of the last amps Nelson Pass worked on before leaving Threshold. The amp is in mint condition thankfully as I bought it on the used market.

But what are the differences in sound quality and construction of a Class A or A/B amp?
Is a Class A amp suppose to be smoother/ lease grainy then a Class AB.
It does run hotter than any other amp I’ve owned.

I fired it up and it sounds good so far that I can tell. But I still need a second amp to drive my bass cabinets before I can get the full sound of the system.

Just looking to have some education on Class A amp design. 

I’m hoping 50 watts is enough to drive my MTM’s. This was a concern. The MTM’s are 92 db efficient.

I got a mint used unit for $800.00, NEW IS $2500.00. So far I think I did good. Just looking for your thoughts.

I had to sell off my tube preamp, and both my amps due to a job layoff at the beginning of the year.
Now I'm back and just purchased my first piece to get up and running again.

Here are pics of the loudspeaker. The top MTM's will must likely be run full range for now bypassing the active EQ.
The bass cabinets will be running thru an active x-over only crossed at about 125HZ.

Here's some pics of the speakers I'm using (DIY) and a couple pics of the Threshold T50 AMP.
Sorry if I rambled on.










For now I can only get a stock photo of the insides until I take it apart.




Thanks,
Steve

[srb]

Class A amplifiers are biased such that the output devices are always on and are generally regarded as having a cleaner and smoother sound by virtue of the elimination of any crossover distortion as the AC waveform transitions to 0V and reverses polarity.

A number of highly regarded Class AB amplifiers are biased higher than than typical (but still a Class AB amplifier) and have more wattage in Class A before transitioning to Class AB.  The compromise in a Class A amplifier, as you noted, is the relative high power consumption and subsequent heat output as they are near maximum power consumption even in a no-signal idle state.

Based on your speakers' 92dB sensitivity, you can plug in your listening distance and other conditions into an SPL Calculator and get an idea of what peak SPL you can expect.  Based on 92dB sensitive speakers and 50W amplifier power, you can expect ~ 102dB peak output at 10 foot listening distance.  It's a ballpark and your actual room acoustics, speaker placement and listening distance will determine the actual peak output.  Adding in your 125Hz bass cabinets will also increase that number.

Steve

[G Georgopoulos]

The main advantage of class-a is low distortion,that's their advantage,however class-a with mosfet o/p devices do have some distortion issues because they are less linear than transistors,transistors on the other hand suffer from thermal distortion.

Because of the heat,lot's of other problems may arise over time

Very Good amplifiers indeed 

[FullRangeMan]

No surprise transistors suffers from bad sound, they are made from sand.
Some call it silicon.

[WireNut]

Awhile back I remember Frank Van Alstine stating that a components worst enemy was heat. Ever since that day I've been leaving the tops off of my equipment to keep down the heat and let it breath.

Since I just purchased a Threshold T50 Class A amp that runs hot, I called threshold and ask if I should leave the top of the Threshold T50 amp and I was told it is not recommended because the Class A  amp would never reach it's operating temperature. Hmm this makes since. What do you all think. If the amp has to run at a certain
temperature, and If I leave the top off in may never see that temperature and keep running and possibly overheat from running constantly.

What's your opinion?

Steve

[Steve]

Since I just purchased a Threshold T50 Class A amp that runs hot, I called threshold and ask if I should leave the top of the Threshold T50 amp and I was told it is not recommended because the Class A  amp would never reach it's operating temperature. Hmm this makes since. What do you all think. If the amp has to run at a certain
temperature, and If I leave the top off in may never see that temperature and keep running and possibly overheat from running constantly.

What's your opinion?

Steve

Absolutely correct Steve. I and probably all designers design around temperature stability. As the amp warms up, the sonics definitely change until stabilization occurs. Same thing happens in SS amplifiers.  When the component stabilizes, the design is tweeked for best, natural sonics.

The down side of course is that some parts, such as electrolytic capacitors have their life shortened. Usually, for example electrolytics, it is about half life for every 20 degrees F increase in temperature, unless modern science has altered this approximate figure.

Cheers
Steve

[Early B.]

Awhile back I remember Frank Van Alstine stating that a components worst enemy was heat. Ever since that day I've been leaving the tops off of my equipment to keep down the heat and let it breath.

At one point, my amp was running hot to the touch, so I removed the top plate and replaced it with a glass top with felt feet that allows some of the heat to escape. Due to changes in placement, cabling and speakers, this is no longer necessary, but it looks cool to be able to see inside my amp.

[G Georgopoulos]

Thermal stability of heatsinks takes say around half an hour or more,however the bias is ready from switch on,so you can listen to it straight away,to get the longer possible life out of it,treat it with care,switch off when you're not listening.

[FullRangeMan]

WireNut,
My opinon worth little, there is experiencied experts here.
But as Steve and George have put clear, your interests are not the same as the factory interests.
My 2 cents

[DTB300]

Thermal stability of heatsinks takes say around half an hour or more,however the bias is ready from switch on,so you can listen to it straight away,to get the longer possible life out of it,treat it with care,switch off when you're not listening.

With my Plinius (switched into Class A) it does take about an hour before it settles in.

The best thing about the Plinius is having the choice of when to run in Class A and when to run in A/B.  This way the amp is always on (in A/B mode), and when I have a few hours to listen to tunes, I "flip the switch" and go into Class A.

In A/B mode it is a nice sounding SS amp, but in Class A, it takes that step up to better sound - as someone mentioned above - "..having a cleaner and smoother sound...".