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 Post subject: Low end roll off performance on amplifier?
PostPosted: Aug Tue 11, 2020 8:04 pm 
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Joined: Nov Thu 04, 2010 5:49 am
Posts: 656
Location: Albany, NY
Hello,

I am currently working on finishing up an amplifier using this circuit:

http://www.hi-ho.ne.jp/ux-45/smokey.html

It *seems* a bit bass shy. I've been using http://sim.okawa-denshi.jp/en/CRtool.php to calculate the roll off point for the RC combinations and I've arrived at the following results:

C1/R5 = 0.78hz

C5/R2 = 7.2hz

I'm quite new at this so please excuse any errors. Also, how does the input potentiometer affect tonality? Does frequency response change depending on the load that the device connected to the input of the amplifier presents to it?

I plan on using the amplifier mostly with low impedance sources such as tablets, mp3 players and tube circuits that are terminated by an output transformer that expects an 8 ohm load on the output.


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 Post subject: Re: Low end roll off performance on amplifier?
PostPosted: Aug Tue 11, 2020 8:29 pm 
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I would expect the low end response to be limited by the output transformer. Tell us its specs / parameters.

Other:
C1-R5 has a time constant of 0.103 sec. That makes the corner frequency about 1.5Hz.

C5 is the decoupling for the 1S5 screen voltage---i don't think that's relevant to the overall frequency response. C6, however, is relevant but I still think the OPT is the dominant issue.

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 Post subject: Re: Low end roll off performance on amplifier?
PostPosted: Aug Tue 11, 2020 8:49 pm 
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Joined: Nov Thu 04, 2010 5:49 am
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Location: Albany, NY
I am currently using it with a Hammond 144Q transformer which, taken from the Hammond website, has a frequency response of "150 Hz. to 15 Khz. (+/- 1 db, reference 1000 Hz.)"

I understand that 150hz isn't exceptionally low in bass extension but, even so, the amplifier does seem a bit tinny.

Now, the primary of this transformer is 10k whereas, according to the data sheets, 3S4's like to see around 5-8k depending on plate voltage. I am using this amplifier to mainly drive headphones that have a 60-64 ohm impedance. Would this mismatch, combined with the higher than expected impedance of the driven load, cause a significant shift upward in frequency response?

Is the 500k potentiomer also playing a part in this given how the load of the input device (tablet, MP3 player, tube circuited with an 8 ohm secondary output transformer) is generally low?


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 Post subject: Re: Low end roll off performance on amplifier?
PostPosted: Aug Tue 11, 2020 9:16 pm 
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Location: Seattle, WA 98122 (from Hungary)
To hit fully flat frequency response all the way down to 150 Hz, I think you'd need to employ some sort of negative feedback from the voice coil to the first amplifier pentode. It's quite difficult to achieve it though with this tube set, due to the cathodes of both tubes being interconnected because of the filament situation. I haven't seen any good example of it even though the circuit is literally in every battery operated AA5 radios (1U5/1S5 and 3S4/3V4).

Without negative feedback, the sound of the amp will always be governed by the impedances defined by combination of the output transformer and the speaker (if all other caps/resistors are right). The speaker's impedance curve alone isn't flat 4 ohm throughout the audible frequency range, but rather quite all over the place between 3.2 and 30 ohms (maybe even more). The transformer adds its own color into this picture.


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 Post subject: Re: Low end roll off performance on amplifier?
PostPosted: Aug Wed 12, 2020 5:50 am 
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I would agree with pixellany. This is from the Radiotron Designers Handbook, 3rd edition:
Attachment:
RDH3-Output-Xfrmr-Inductance.jpg
RDH3-Output-Xfrmr-Inductance.jpg [ 235.31 KiB | Viewed 659 times ]

John

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 Post subject: Re: Low end roll off performance on amplifier?
PostPosted: Aug Wed 12, 2020 6:28 am 
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Joined: Dec Sun 14, 2008 3:33 pm
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Location: Tokyo
You could try boosting the bass by cutting the treble and midtones. To start with, try a .0047~.047uF cap in series with a 1M resistor, that combination connected across R3, 1.2M, the preamp plate load resistor. You'll have to experiment with the cap values.

This does cut treble/midtone gain but you compensate by turning up the volume and the practical effect is a bass boost.

Rob


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 Post subject: Re: Low end roll off performance on amplifier?
PostPosted: Aug Wed 12, 2020 4:02 pm 
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Joined: Nov Thu 04, 2010 5:49 am
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Location: Albany, NY
I sincerely appreciate all of your input. I did a bit more testing last night and it seems like most of the problem was due to the impedance mismatch from the output transformer that I was using which has a primary impedance of 10k versus the 5k that the 3S4 is really looking for. I had a transformer on hand with a 4k primary that I used to test with and now the amplifier sounds much more balanced. A very small improvement came from switching the potentiometer from 500k to 100k. The treble response became just a bit less and drive requirements from input sources definitely improved.

One question I have is, if you have a transformer with a 4k primary and an 8 ohm secondary and you apply a 16 ohm load to the secondary, does that mean that the output tube now sees an 8k impedance on the primary?


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 Post subject: Re: Low end roll off performance on amplifier?
PostPosted: Aug Wed 12, 2020 4:50 pm 
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correct...a transformer has a fixed impedance ratio, which is the square of the turns ratio.

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 Post subject: Re: Low end roll off performance on amplifier?
PostPosted: Aug Thu 20, 2020 5:14 pm 
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Location: Rochester NY USA
A pentode has a high output impedance, so the inductance of the transformer is the limiting factor for low-frequency response. (likely your 5K transformer has more inductance). Wiring the output stage as a triode (connect screen to plate) or using plate-to-grid feedback to give a lower output impedance and improve low-frequency response. For the latter, try a 3.3 meg resistor from the plate of the 3S4 to the plate of the 1S5, and increase the 1S5 plate resistor to 1.5 or 1.8 meg.

Triode mode will reduce gain and max output power. Plate-to-grid feedback will reduce gain without reducing max output.

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 Post subject: Re: Low end roll off performance on amplifier?
PostPosted: Aug Tue 25, 2020 6:46 pm 
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Perhaps find a transformer with a tap for operating the output tube in ultralinear mode. That may help bass response.


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 Post subject: Re: Low end roll off performance on amplifier?
PostPosted: Sep Mon 28, 2020 2:29 pm 
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Maybe this is too obvious but ... the speaker itself is also a limiting factor in overall frequency response. So is the "case" it's in, such as it is. You can maximize the component's frequency response both high and low and you'll still be limited by the speaker itself.

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 Post subject: Re: Low end roll off performance on amplifier?
PostPosted: Sep Wed 30, 2020 2:04 pm 
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And how does the SIZE of the transformer, ie more steel, affect low response? Just by saturating, if too little, but not the main factor in lowest response? I get the impression that at a certain point in the past there was not much concern about any response below about 50 Hz, due to the limitations of musical source materials. And loudspeakers.

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 Post subject: Re: Low end roll off performance on amplifier?
PostPosted: Sep Wed 30, 2020 2:13 pm 
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Posts: 398
Bigger transformer size? Then it won't saturate easily and AF distortion would be low and frequency response wide. The inductance should be large enough to make sure that it does not limit the low frequency response

Some principles of radiotelephony 4; Designing a modulator https://www.robkalmeijer.nl/techniek/el ... index.html


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