Pass Amps- JUst insensitive or is something wrong with my ideas and builds?

[VictoriaGuy]

From memory F6 has a gain of around 5.
To drive the amp to clipping you will need a preamp with gain of at least 6dB.

If you decide to go with the preamp with gain route, then Zenmod has a very nice preamp you could build.

I would suggest a preamp, BA-3 pre project is a perfect diy project with a board from our store. For a scratch build I really like the LSK pre, both Juma's and the one presented at the Burning Amp Festival by Nelson. I own all 3.

The ba3 preamp can run off your existing 24/-24 psu. A jfet boz can be built with batteries. Lots of options. Even a cheap eBay ic preamp will get you some gain.

Thanks for the preamp suggestions- I'll do some reading and pick/build a design.
(I have a BOSOZ -builta couple of weeks ago- but I don't feel it's up to the job requirements. , and the B1 boards are still on the shelf here -though the jfets I bought for the B1s are being used for some of these other amp projects....)

 

[Soundhappy]

It's not 'sound quality' but gain that's the issue....
It would be very useful if somebody would produce a 'summary table' with the basic gain info for the Pass circuits. The power amps don't compare with other diy or commercial power amps in gain, and some of the very popular Pass 'line stages' (B1,BOSOZ) don't have enough gain to do the job either.

You don't read this page ?

FIRST WATT PRODUCTS

Funny

 

[VictoriaGuy]

You haven't made a mistake.
Just double the gain of the amp.
Change the 100 Ohm feedback resistor to 200 Ohms and see if that is to your liking.
Try that first.

THANK YOU!

That's definitely an easy experiment to try right now.
Just to be clear - the feedback resistor is the R3 100 ohm between the output and the primary of the transformer?

And, it sounds like I won't need to double the gain, just add something like 5-8 dB (??) so perhaps an intermediate value of R3 will do the job?

I have some 3W low resistance pots which I can use to experiment with this...

 

[Soundhappy]

@ VictoriaGuy

How much of gain from F6 ?
You know now i hope
F4 have gain of 0dB

 

[VictoriaGuy]

You don't read this page ?

FIRST WATT PRODUCTS

Funny

I probably read it but didn't fully understand the implications of the data there...

Perhaps adding another column would have helped a neophyte:
"Requires a preamp with 5-10dB gain for use with CD player or line source-Y/N"


I'm confused about the whole issue of input levels.
Most CD players have a nominal output of '2 V' listed in the Specs. This is peak to peak?
Mr Pass states that the input sensitivity of the F6 for full power output is '2.83 volts'. Is this RMS or p-p?
My F6 measurements show an output of 2.2 watts with a 2 V p-p input sine wave, so I think that the CDP manufacturers are using p-p volts and Mr Pass is using RMS volts.
.....but I may well be confused

 

[VictoriaGuy]

@ VictoriaGuy

How much of gain from F6 ?
You know now i hope
F4 have gain of 0dB

You will need to look hard to find dB gain figures for some common (Yamaha, Rotel, etc) commercial power amps, so it is difficult to compare, for a beginner.

A simple statement: "Needs a preamp with 5dB gain for ordinary use with a CD player" would not be difficult to add to the specs and discussions.

 

[VictoriaGuy]

@ VictoriaGuy

How much of gain from F6 ?
You know now i hope
F4 have gain of 0dB

I get your point; thanks.

There's also more to the issue of 'input sensitivity' than the overall gain of the amplifier in dB, as far as I know.

 

[Nelson Pass]

I use rms volts unless otherwise stated. Peak will be 1.41 times that,
and p-p is 2.83 times.

To be sure, it's easier to get quality out of simple amplifiers if you ask
for less gain, and I'm an old fart listening to quite efficient speakers at
low levels.

 

[analog_sa]

And, it sounds like I won't need to double the gain, just add something like 5-8 dB

Nice one

 

[VictoriaGuy]

I use rms volts unless otherwise stated. Peak will be 1.41 times that,
and p-p is 2.83 times.

To be sure, it's easier to get quality out of simple amplifiers if you ask
for less gain, and I'm an old fart listening to quite efficient speakers at
low levels.

Thanks for adding that info - I'll try to keep the 'volts thing' straight in the future.

BTW, I do want to emphasize that I've really enjoyed building your Pass DIY designs in the past few weeks -BOSOZ,ACA, and F6. The low parts count means that I have some chance (helped by watching the YouTube videos of your BurningAmp talks) of understanding how they work.
I also really appreciate the fact that (so far) the designs for the ACA and the F6 are quite insensitive to bad habits like oscillation, motorboating and hum, since I've used either diy PCBs or just P2P wiring (on the F6), and certainly have not paid a great deal of attention to 'best practices' on grounds and layout.

Thanks!
(For me, it's more fun to build stuff as much as possible 'from scratch' rather than populating a well-designed PCB, but the results aren't always so good...

 

[fabrice63]

(For me, it's more fun to build stuff as much as possible 'from scratch' rather than populating a well-designed PCB, but the results aren't always so good...

You reap what you sow, man

.

 

[VictoriaGuy]

You reap what you sow, man

.

Yup- that's why it's called a 'hobby' not 'a business' for me.....

 

[2 picoDumbs]

THANK YOU!

That's definitely an easy experiment to try right now.
Just to be clear - the feedback resistor is the R3 100 ohm between the output and the primary of the transformer?

And, it sounds like I won't need to double the gain, just add something like 5-8 dB (??) so perhaps an intermediate value of R3 will do the job?

I have some 3W low resistance pots which I can use to experiment with this...

Yes.
Go for your life, play around then replace pot with the nearest fixed resistor, or you can leave the pot in place for adjustable gain.
A 100 Ohm fixed resistor in series with a 100 or 200 Ohm pot would be a suitable place to start to experiment.

 

[VictoriaGuy]

Yes.
Go for your life, play around then replace pot with the nearest fixed resistor, or you can leave the pot in place for adjustable gain.
A 100 Ohm fixed resistor in series with a 100 or 200 Ohm pot would be a suitable place to start to experiment.

I'll report back here if I get any measurements of interest.

I'm still thinking about (and checking my parts collection for) preamp ideas as well.

 

[moe29]

build an Aleph 2

 

[VictoriaGuy]

build an Aleph 2

Well the 26dB of gain is appealing , but the weight,power consumption, and power output...not so much!
I'm not very practiced at using dB, but isn't a 100W 26dB amp equivalent to a 20dB 25 watt output amp, when thinking about how much input voltage is required to drive the amp to maximum output?
So, my goal really is to get the F6 (or similar) amp to get closer to that 20dB gain number - either by sticking a preamp in front of it, or by playing around with the feedback....that's how I see it anyway..but I'm open to being corrected

 

[alvinlim]

Well the 26dB of gain is appealing , but the weight,power consumption, and power output...not so much!

I'm not very practiced at using dB, but isn't a 100W 26dB amp equivalent to a 20dB 25 watt output amp, when thinking about how much input voltage is required to drive the amp to maximum output?

So, my goal really is to get the F6 (or similar) amp to get closer to that 20dB gain number - either by sticking a preamp in front of it, or by playing around with the feedback....that's how I see it anyway..but I'm open to being corrected

A 26dB amp has higher gain than a 20 dB amp. A 26dB amp is louder. But depending on the sensitivity of your speakers, a 100w 20dB amp is less prone to clipping than a 25w 20dB amp. Definitions can be found here:

https://en.m.wikipedia.org/wiki/Amplifier_figures_of_merit

The nice thing about class A Pass amps is that they clip 'softly' - which do not sound hard to the ears. I am simplifying a lot here, but you get the gist





Sent from my iPad using Tapatalk

 

[Hikari1]

Victoriaguy,

How loud do you listen and what speakers are you using?

If you ever make it down to Seattle I'll show you how hard an F5 can drive a set of 89db 3 ways with a BA-3 preamp or the Salas Dcg3 preamp or even a buffer if you are interested.

 

[VictoriaGuy]

If you ever make it down to Seattle I'll show you how hard an F5 can drive a set of 89db 3 ways with a BA-3 preamp or the Salas Dcg3 preamp or even a buffer if you are interested.

Thanks for the offer; I appreciate it.
The buffer (no gain) demo would be interesting.
The whole point of my complaint was that the F6 seemed to need a preamp to perform at its best with a 'line level' (aka CDP) input- and it sounds like your F5 is the same ?

I have some sensitive speakers (AudioNirvana 95dB+ ??) but the ones I'm listening to most these days are less efficient - I like to be able to play with whatever bargains I drag home!

And, I don't listen very loud, actually....mostly jazzy and bluesy stuff with some chamber music tossed in. My house is small.

 

[VictoriaGuy]

I'll report back here if I get any measurements of interest.

As promised....
(I don't know if this will be interesting to anybody else, but it was to me. )

I have an HP339 Distortion Analyzer which I generally use just as an oscillator, but I got some THD numbers with it. I wouldn't put much confidence in the absolute numbers (the HP339 hasn't been calibrated in years) but the changes in %THD were as I expected, and possibly of interest. "It's all relative" as the saying goes...

The p-p voltages were measured using a scope.
Input signal was 10kHz sine wave, 2 V p-p.

The idea was to change the value of R3- the 100 ohm 'feedback resistor' to observe the changes in gain and THD.

As built, with Rfb= 100ohms, output into an 8 ohm dummy load was 12V p-p
15.5dB gain THD= 0.03%

Rfb=200 ohms Output was 17 V p-p
18.5 dB gain THD=0.06%

Rfb=300 ohms Output was 24 V p-p
21.5 dB gain THD=0.14%

I'll need some sort of 'volume control' (aka input attenuator) in front of the F6 anyway - I was using a 10k pot for some quick tests with music this evening.
Since I easily can build a simple preamp with very low THD numbers (with volume and balance control), for me it makes sense to go in that direction.

That said, if the 0.14% THD number is correct, that's not bad, so boosting the gain by increasing R3 may be a useful possibility to keep in mind.
Thanks to 2Pico for that idea!

BTW, the F6 sounds good, when I get it loud enough!