It's seems the complimentary output stage for ba3 doesn't require matching output devices but iirc the f4 did. I think these are very similar output stages.
Is it the case that actually its not essential for either but its encouraged and was so in whichever article I read about the f4, or due to slight variances in resistor values and the like, the ba3 isn't as susceptible.
Is it the case that actually its not essential for either but its encouraged and was so in whichever article I read about the f4, or due to slight variances in resistor values and the like, the ba3 isn't as susceptible.
The BA complimentary output stage and the F4 are identical save for the Jfet buffer.
BA- output stage is shown on the schematic with 1ohm source resistors (instead of .47ohm) to help with looser matches.
Matching helps either amp.
BA- output stage is shown on the schematic with 1ohm source resistors (instead of .47ohm) to help with looser matches.
Matching helps either amp.
I built a F4 along with a BA3 pre and have enjoyed that combination for some time. I have a pair of JBL L-220 speakers that I purchased new in the mid 70's. They have a system sensitivity of 90db. I updated the crossover components before adding the F4 combo to the mix.
So I finally completed another F4 to allow mono balanced output. Wow really love what a pair of F4 amps can deliver to those speakers. I have a F6, but the F4 combo is still my favorite. Next up is an M2.
Thank you Mr. Pass
So I finally completed another F4 to allow mono balanced output. Wow really love what a pair of F4 amps can deliver to those speakers. I have a F6, but the F4 combo is still my favorite. Next up is an M2.
Thank you Mr. Pass
Hi guys,
After a long period of life getting in the way, I am back working on my Impass/F4.
I am curious about how stable the F4 is for bias current and DC offset, once you have it set correctly? More specifically, would it be possible to replace the potentiometers with a fixed resistor, once the correct value for the amp was determined?
The reason for my question is that I have found some data that shows current noise of a cermet pot is roughly an order of magnitude higher than a fixed metal film resistor of the same value. At a minimum, it might be useful to use a smaller value pot in series with a fixed resistor. Any thoughts?
Jac
After a long period of life getting in the way, I am back working on my Impass/F4.
I am curious about how stable the F4 is for bias current and DC offset, once you have it set correctly? More specifically, would it be possible to replace the potentiometers with a fixed resistor, once the correct value for the amp was determined?
The reason for my question is that I have found some data that shows current noise of a cermet pot is roughly an order of magnitude higher than a fixed metal film resistor of the same value. At a minimum, it might be useful to use a smaller value pot in series with a fixed resistor. Any thoughts?
Jac
The problem you may run into is the pot value may be something crazy like 3.56kΩ......
Good luck finding an off the shelf resistor.
Not to mention the complementary resistor to form the divider.
Maybe a wirewound pot or a CP trimmer hung off of a fixed value.
But all of this might be in vain if there is no sonic impact.
Good luck finding an off the shelf resistor.
Not to mention the complementary resistor to form the divider.
Maybe a wirewound pot or a CP trimmer hung off of a fixed value.
But all of this might be in vain if there is no sonic impact.
Would 3.57k be close enough?
CMF553K5700FKEK Vishay / Dale | Mouser
But seriously, your point is taken.
The sonic value is the real question. I tend to go off researching something of interest, only to have to decide later if it was worth the trip.
I am still curious whether these pots need to be reset over time. If the amp changes set points over time, then the question is totally moot. If it doesn't need to be reset, then I have to decide if it's worth the experiment is even worth doing.
Jac
Hi mrdave45,
I was replying to Einric as you were posting. Thanks a lot. That answers the question and, based on your info, it's not worth the experiment.
Jac
Pass DIY Addict
Joined 2000
Paid Member
Maybe then, it's most sensitive between cold and warm rather than warm and hot. Although I do think I need to reset mine. It's in a 4u Hifi2000 deluxe and it does vary how hot the heat sinks feel depending on time of year.
Being scientific and all I should really run the experiment but that does require decipline and no tweaking and all that.
Being scientific and all I should really run the experiment but that does require decipline and no tweaking and all that.
Not served. Just a person who likes the way his system sounds and is enough of a perfectionist to check the details. I admit that I am probably too lazy to adjust them with temperature, but I appreciate hearing about your experience. I do have a tendency to chase down dead end rat holes for very little benefit.
Thanks.
Jac
You could use a 3k3 metal resistor with low noise and low drift.
Then add on a 500r to use as a fine trim.
The drift of the 270ohms of the 500VR is a lot less than the drift of a 5kVR.