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F5 Turbo Builders Thread
F5 Turbo Builders Thread
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Old 25th February 2018, 02:35 AM   #5081
KyleMD is offline KyleMD  United States
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Join Date: Jun 2016
Location: Chicago
Well, I found the short. I must have tightened the transformer bolt too tight, as there was a tear in the wrapper that exposed the copper to the chassis. Once I insulated the tear, the amp passed the lightbulb test and I was able to set the bias. The 2200 ohm r5/r6 resistor change appears to have been spot on. I was able to set initial bias 220 ohms with plenty of turns left on the pots, and it's cooking now. After two hours I'll up it to 320.
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Old 25th February 2018, 06:42 PM   #5082
KyleMD is offline KyleMD  United States
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Default Hum Breaking Resistor (HBR) Location

Alright, I've successfully set the bias to 320mV on the amplifier and it's working...for the most part!

With inputs disconnected from a source, the amp does appear to have a very slight 60 hz hum. It's only noticeable if I put my ear right up next to the drivers on my speakers, and even then, I need to listen for it. Frankly, it's something I can live with and it's not something I'm all that worried about.

When I connect a source to the inputs, however, I'm getting a buzz that's more audible. I don't believe it's an AC ground loop issue because the buzz occurs even when when I connect the amp directly to a battery-powered source (i.e. phone, laptop). The buzz is also consistent and doesn't rise or decrease in level when I change the volume on the source, so I don't believe it's a signal interference issue.

I did some reading in the F5 thread and came across this post regarding cross-channel ground loops and how to minimize them with a hum breaking resistor (HBR) on the F5 boards:

F5 power amplifier

I think the cross-channel ground loop may be my issue and I would like to try adding the HBR to my F5T 3.0 boards. I believe the correct location for the resistor would be between the input ground and the link pad shown in this photo from the build guide:

Click the image to open in full size.

Is this correct?
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Old 28th February 2018, 12:46 PM   #5083
KyleMD is offline KyleMD  United States
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I went ahead and added 2 ohms of resistance (3W rated) at the location shown above, with the signal ground connected to the Link pad. It completely eliminated all hum and buzz! The amp now sounds phenomenal. The jfets from punkydawgs are the real deal, although they did require the R5/R6 resistor change to set the bias correctly.

I'm going to go ahead and call this project complete.
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Old 15th March 2018, 02:16 AM   #5084
Keremito is offline Keremito  United States
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Default ground loop noise and class a power.

Thanks Kyle, I've been chasing that noise problem for some time, did the same, removed the link and put a 2.2 ohms, 2 Watts resistors. Now it's dead quiet even using non shielded interconnects.
I have a question for the guys who know.. While outside the system, on the bench I connected I function generator to the amp through a preamp. It clipped at 27 volts end into a 6.5 ohms resistor, that is 112 Watts. This is a V2 running at +/- 43 volts at the PS output with load, devices are biased at 0.350 you volts=0.7 amps so total bias is 2.8 amps. From these numbers, how do I get to the power number where the amp leaves class A to become class AB? In other words what is the class A output of the amplifier.
Thanks for your kind answers.
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Old 15th March 2018, 04:08 AM   #5085
Erlend Sæterdal is offline Erlend Sæterdal  Denmark
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F5 Turbo Builders Thread
Good question wich I sm following close. Have asked that kind off question on other forums. 40 answers is all different.
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Old 15th March 2018, 04:39 AM   #5086
Hikari1 is offline Hikari1  United States
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Nelson says in the f5 turbo article that 2A bias will give you 100w class peak into 8ohms. I am assuming half into 4 ohms. But I don’t know if any math or calculations.
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Old 15th March 2018, 06:00 AM   #5087
Keremito is offline Keremito  United States
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Default Leaving class A

Nelson says in that article: "Push-pull amplifiers generally operate in Class A mode up to a point where the output current is twice the value of the bias current. In the Class A region, both halves of the circuit share the signal simultaneously. Beyond that the signal is handled solely by the push (+) half of the amplifier or the pull (-) half."

Am i correct thinking that if the output transistors have 0.7 A rms of bias, then they will stay in class up to twice that number? I=1.4 A rms
Somewhere in the forum I read that there is usually a loss of up to 5 volts in the amplification stage. The rail voltages being +/- 43, minus the voltage drop: 38 Volts x 1.4 amps = 53.2 watts of class A.
Is this correct?
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Old 15th March 2018, 06:45 AM   #5088
AudioSan is offline AudioSan  Norway
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Quote:
Originally Posted by Keremito View Post
Thanks Kyle, I've been chasing that noise problem for some time, did the same, removed the link and put a 2.2 ohms, 2 Watts resistors. Now it's dead quiet even using non shielded interconnects.
I have a question for the guys who know.. While outside the system, on the bench I connected I function generator to the amp through a preamp. It clipped at 27 volts end into a 6.5 ohms resistor, that is 112 Watts. This is a V2 running at +/- 43 volts at the PS output with load, devices are biased at 0.350 you volts=0.7 amps so total bias is 2.8 amps. From these numbers, how do I get to the power number where the amp leaves class A to become class AB? In other words what is the class A output of the amplifier.
Thanks for your kind answers.
You say this is a v2. so 2 output pairs? in that case you have 1.4A bias. around 30W class A @8ohm. and 15W class A @4ohm.
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Old 15th March 2018, 06:57 AM   #5089
Keremito is offline Keremito  United States
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Quote:
Originally Posted by AudioSan View Post
You say this is a v2. so 2 output pairs? in that case you have 1.4A bias. around 30W class A @8ohm. and 15W class A @4ohm.
Thanks.
P = 2 I² R = 2 x 1.96 x 8 = 31.36 watts Max class A
Correct this time?
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Old 15th March 2018, 07:06 AM   #5090
AudioSan is offline AudioSan  Norway
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That is correct.
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