The Delta 4-7 is a limited production dual preamp based on the famous British microphone preamp of the 60’s. These are all hand wired point to point, transformer balanced input and output, EF86 gain with 6DJ8 output drive. Literally the best. Only 20 of these will be made under Highland Dynamics PLUS.
This preamp circuit always fascinated me with its 3D like sound. John Baccigaluppi has a version of this preamp made by Revolution Recording and I loved it!, really everything I used it on, my favorites were vocals, overhead drums and bass, but really anything. There were some things I didn’t like about it, the input gain was strange to me. The variable input pad made it all work. For this project I used specific components based on trial, all paper and oil capacitors by West-Cap, Vitamin Q and NYT and 2 watt carbon resistors used in the audio path, precision 1% resistors used elsewhere. Grayhill rotary switches for gain and bass cut. This is all hand wired with solid core coper wire, large film capacitors used for amplifier power delivery.
Only 20 of these will be made under Highland Dynamics PLUS.
Input impedance: select from 150 or 37.5. These are two different taps on the input transformer. Handy for matching different microphone transformers and also gain and tone difference.
Input pad: The input transformer can handle full line level input, so this pad is placed after the input transformer and unbalanced D.I. giving you variable pad from no pad to line level input. Handy for adjusting input levels on the fly and different D.I. inputs. I think of this as almost a gain control.
Output pad: This is after the output transformer allowing you to set the output level if its to hot.
Bass cut: This gives you 3 different high pass filters, from around 60 Hz up to 300 Hz. Placed after the input transformer and D.I. and before any gain stages.
Gain control: The gain control in this circuit works on sending different amounts of feedback into the EF86. There are four gain settings: Low, Med, High and Open+ for no feedback applied. This works well but also gives you different tonal qualities, Low is the most HiFi and controlled and Open+ with no feedback and the most colorful and character. I use the variable Input Pad and Output to control the gain if I like a particular gain setting for tonal reasons.