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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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It is currently 22 Sep 2019, 08:09

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PostPosted: 27 May 2019, 21:19 
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Joined: 27 May 2019, 20:40
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I've been working on a "cigar box" style amp for the past couple of months and I need some help on the output stage. I had a spare 20v power adapter and I decided to build an amp around this for my sax. I play with a band, and wanted to have a small amp to help fill in the sound. It is designed for a dynamic mic that produces 2~3mv/Pa (schematic attached). The problem is in the voltage gain stage. I converted a CMOS inverting amp design to BJTs. It works okay but it can only drive a high impedance loads - that is why I have it loading a MOSFET before the current drive stage. However I am losing 2-3 volts at the Gate/Source junction to "turn on" the MOSFET. Is there a better way to buffer the output signal before the current stage. I've rebuilt the amp using Opamps in the voltage gain sections and while it works great - it feels like cheating... I really want to build this with discrete components.

Ryan


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PostPosted: 30 May 2019, 04:37 
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Joined: 23 Feb 2017, 02:02
Posts: 546
Plorbem here is that the single transtor phase shifter is satuarating and I had this once to try and get some drive to do R&D.

This phase shifter is only really applicable for hv applications, some how valve amps get it working.
But low voltages this one is impossible.


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PostPosted: 30 May 2019, 04:54 
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Joined: 23 Feb 2017, 02:02
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Don't be offended with red ink.
I've changed to red ink only for schematics for a long time already

This is a diffrential pair with other input grounded via capacitor so you can get some diffrential action (a high and low going signal, inverted & non inverted)

Use extra buffering if you want burtual drive


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PostPosted: 30 May 2019, 08:31 
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Joined: 23 Feb 2017, 02:02
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To reduce the gain you can also try to put a resistor in series with the 10u on the other half of the diffrential pair which goes from base to ground
Otherwise using k ressitors will reduce linearity but you need to rebias if the resistor values large, simply you can replace 470r with pot or trial and error


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PostPosted: 04 Jun 2019, 02:09 
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Joined: 23 Feb 2017, 02:02
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using ressitors K to reduce gain, will increase linearity of the gain stage, but you need to rebias if the resistor values large


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PostPosted: 05 Jun 2019, 08:00 
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Joined: 06 Jun 2008, 18:23
Posts: 5136
Location: Australia
Not sure the tin is a cigar box.

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Projects:"retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp | ”najah” - Raw 180W Tripath Class D power amp | "Icon" - Shuguang CV-181Z preamp


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